diff --git a/.github/workflows/bamboo_smtp.yml b/.github/workflows/bamboo_smtp.yml
new file mode 100644
index 0000000..e202c09
--- /dev/null
+++ b/.github/workflows/bamboo_smtp.yml
@@ -0,0 +1,59 @@
+name: Bamboo SMTP
+
+on: push
+
+jobs:
+  test:
+    name: Test on Elixir ${{ matrix.elixir }} / OTP ${{ matrix.otp }}
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        elixir: [1.7.4, 1.8.2, 1.9.1, 1.10.3]
+        otp: [19.3, 20.3, 21.3, 22.0]
+        exclude:
+          - elixir: 1.8.2
+            otp: 19.3
+          - elixir: 1.9.1
+            otp: 19.3
+          - elixir: 1.10.3
+            otp: 19.3
+          - elixir: 1.10.3
+            otp: 20.3
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - uses: actions/cache@v2
+        with:
+          path: deps
+          key: ${{ runner.os }}-mix-${{ hashFiles(format('{0}{1}', github.workspace, '/mix.lock')) }}-elixir-${{ matrix.elixir }}-otp-${{ matrix.otp }}
+
+      - uses: actions/setup-elixir@v1
+        with:
+          otp-version: ${{ matrix.otp }}
+          elixir-version: ${{ matrix.elixir }}
+
+      - run: mix deps.get
+      - run: mix compile --warnings-as-errors
+      - run: mix credo --strict
+      - run: mix test
+
+  doc:
+    name: Generate inch report
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - uses: actions/cache@v2
+        with:
+          path: deps
+          key: ${{ runner.os }}-mix-${{ hashFiles(format('{0}{1}', github.workspace, '/mix.lock')) }}
+
+      - uses: actions/setup-elixir@v1
+        with:
+          otp-version: 22.0
+          elixir-version: 1.10.3
+
+      - run: mix deps.get --only docs
+      - run: MIX_ENV=docs mix inch.report
diff --git a/.travis.yml b/.travis.yml
deleted file mode 100644
index 7c66d9d..0000000
--- a/.travis.yml
+++ /dev/null
@@ -1,37 +0,0 @@
-sudo: required
-language: elixir
-elixir:
-  - 1.7.4
-  - 1.8.2
-  - 1.9.1
-  - 1.10.3
-otp_release:
-  - 19.3
-  - 20.3
-  - 21.3
-  - 22.0
-matrix:
-  exclude:
-    - elixir: 1.8.2
-      otp_release: 19.3
-    - elixir: 1.9.1
-      otp_release: 19.3
-    - elixir: 1.10.3
-      otp_release: 19.3
-    - elixir: 1.10.3
-      otp_release: 20.3
-    
-cache:
-  directories:
-    - _build
-    - deps
-script:
-  - mix compile --warnings-as-errors
-  - mix test
-  - mix credo --strict
-after_script:
-  - mix deps.get --only docs
-  - MIX_ENV=docs mix inch.report
-notifications:
-  slack:
-    secure: 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
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 10fb8ef..df67fa2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,27 @@
 # CHANGELOG
 
+## 3.1.0 - 2020-11-23
+
+- Fix for using custom config with `response: true` by bumping `bamboo` version to `~> 1.6` ([#150])
+- Implement our custom test adapter ([#151])
+- Fix CI random failure by attaching FakeGenSMTP Server process to Test supervision tree.([#153])
+- Add Content-ID header when needed([#154])
+- Base 64 encode the headers only when the content contains non-ASCII characters.([#155])
+- Handle `:permanent_failure` exception and re-raising it as a `SMTPError`.([#156])
+- After bumping the dependencies, the project requires([#149]):
+    - credo `~> 1.4.1`
+    - bamboo `~> 1.6`
+    - excoveralls `~> 0.13.3`
+    - gen_smtp  `~> 1.0.1`
+
+[#149]: https://github.com/fewlinesco/bamboo_smtp/pull/149
+[#150]: https://github.com/fewlinesco/bamboo_smtp/pull/150
+[#151]: https://github.com/fewlinesco/bamboo_smtp/pull/151
+[#153]: https://github.com/fewlinesco/bamboo_smtp/pull/153
+[#154]: https://github.com/fewlinesco/bamboo_smtp/pull/154
+[#155]: https://github.com/fewlinesco/bamboo_smtp/pull/155
+[#156]: https://github.com/fewlinesco/bamboo_smtp/pull/156
+
 ## 3.0.0 - 2020-09-10
 
 - Fix eml attachment ([#137]).
diff --git a/README.md b/README.md
index 0a4c9ee..eac6856 100644
--- a/README.md
+++ b/README.md
@@ -1,5 +1,5 @@
 [![Hex pm](https://img.shields.io/hexpm/v/bamboo_smtp.svg)](https://hex.pm/packages/bamboo_smtp)
-[![Build Status](https://travis-ci.org/fewlinesco/bamboo_smtp.svg?branch=master)](https://travis-ci.org/fewlinesco/bamboo_smtp)
+[![Build Status](https://github.com/fewlinesco/bamboo_smtp/workflows/Bamboo%20SMTP/badge.svg)](https://github.com/fewlinesco/bamboo_smtp/actions)
 [![Inline docs](http://inch-ci.org/github/fewlinesco/bamboo_smtp.svg)](http://inch-ci.org/github/fewlinesco/bamboo_smtp)
 
 # Bamboo.SMTPAdapter
@@ -14,7 +14,7 @@ The package can be installed as:
 
   ```elixir
   def deps do
-    [{:bamboo_smtp, "~> 3.0.0"}]
+    [{:bamboo_smtp, "~> 3.1.0"}]
   end
   ```
 
@@ -52,6 +52,14 @@ The *hostname* option sets the FQDN to the header of your emails, its optional,
 
 4. Follow Bamboo [Getting Started Guide](https://github.com/thoughtbot/bamboo#getting-started)
 
+5. **Optional** Set `BambooSMTP.TestAdapter` as your test adapter:
+
+  ```elixir
+ # In your config/config.exs file
+ if Mix.env() == :test do
+   config :my_app, MyApp.Mailer, adapter: MyApp.SMTPTestAdapter
+ end
+ ```
 ## Usage
 
 You can find more information about advanced features in the [Wiki](https://github.com/fewlinesco/bamboo_smtp/wiki).
diff --git a/lib/bamboo/adapters/smtp_adapter.ex b/lib/bamboo/adapters/smtp_adapter.ex
index 665a0c4..20e0b2e 100644
--- a/lib/bamboo/adapters/smtp_adapter.ex
+++ b/lib/bamboo/adapters/smtp_adapter.ex
@@ -71,11 +71,18 @@ defmodule Bamboo.SMTPAdapter do
       config
       |> to_gen_smtp_server_config
 
-    email
-    |> Bamboo.Mailer.normalize_addresses()
-    |> to_gen_smtp_message
-    |> config[:transport].send_blocking(gen_smtp_config)
-    |> handle_response
+    response =
+      try do
+        email
+        |> Bamboo.Mailer.normalize_addresses()
+        |> to_gen_smtp_message
+        |> config[:transport].send_blocking(gen_smtp_config)
+      catch
+        e ->
+          raise SMTPError, {:not_specified, e}
+      end
+
+    handle_response(response)
   end
 
   @doc false
@@ -96,6 +103,10 @@ defmodule Bamboo.SMTPAdapter do
     raise SMTPError, {reason, detail}
   end
 
+  defp handle_response({:error, detail}) do
+    raise SMTPError, {:not_specified, detail}
+  end
+
   defp handle_response(response) do
     {:ok, response}
   end
@@ -191,7 +202,17 @@ defmodule Bamboo.SMTPAdapter do
   end
 
   defp rfc822_encode(content) do
-    "=?UTF-8?B?#{Base.encode64(content)}?="
+    if contains_only_ascii_characters?(content) do
+      "=?UTF-8?B?#{content}?="
+    else
+      "=?UTF-8?B?#{Base.encode64(content)}?="
+    end
+  end
+
+  defp contains_only_ascii_characters?(content) do
+    content
+    |> String.to_charlist()
+    |> List.ascii_printable?()
   end
 
   def base64_and_split(data) do
@@ -215,7 +236,19 @@ defmodule Bamboo.SMTPAdapter do
     |> add_smtp_line(text_body)
   end
 
-  defp add_attachment_header(body, %{content_type: content_type} = attachment)
+  defp add_attachment_header(body, attachment) do
+    case attachment.content_id do
+      nil ->
+        add_common_attachment_header(body, attachment)
+
+      cid ->
+        body
+        |> add_common_attachment_header(attachment)
+        |> add_smtp_line("Content-ID: <#{cid}>")
+    end
+  end
+
+  defp add_common_attachment_header(body, %{content_type: content_type} = attachment)
        when content_type == "message/rfc822" do
     <<random::size(32)>> = :crypto.strong_rand_bytes(4)
 
@@ -225,7 +258,7 @@ defmodule Bamboo.SMTPAdapter do
     |> add_smtp_line("X-Attachment-Id: #{random}")
   end
 
-  defp add_attachment_header(body, attachment) do
+  defp add_common_attachment_header(body, attachment) do
     <<random::size(32)>> = :crypto.strong_rand_bytes(4)
 
     body
diff --git a/lib/bamboo/adapters/test_adapter.ex b/lib/bamboo/adapters/test_adapter.ex
new file mode 100644
index 0000000..5c99743
--- /dev/null
+++ b/lib/bamboo/adapters/test_adapter.ex
@@ -0,0 +1,56 @@
+defmodule BambooSMTP.TestAdapter do
+  @moduledoc """
+  Based on `Bamboo.TestAdapter`, this module can be used for testing email delivery.
+
+  The `deliver/2` function will provide a response that follow the format of a SMTP server raw response.
+
+  No emails are sent, instead it sends back `{%Bamboo.Email{...}, {:ok,"<raw_smtp_response>"}}` 
+  for success and raise an exception on error.
+
+  ## Example config
+
+      # Typically done in config/test.exs
+      config :my_app, MyApp.Mailer,
+        adapter: BambooSMTP.TestAdapter
+
+      # Define a Mailer. Typically in lib/my_app/mailer.ex
+      defmodule MyApp.Mailer do
+        use Bamboo.Mailer, otp_app: :my_app
+      end
+  """
+
+  @behaviour Bamboo.Adapter
+
+  @doc false
+  def deliver(_email, _config) do
+    send(test_process(), {:ok, "Ok #{Enum.random(100_000_000..999_999_999)}"})
+  end
+
+  defp test_process do
+    Application.get_env(:bamboo, :shared_test_process) || self()
+  end
+
+  def handle_config(config) do
+    case config[:deliver_later_strategy] do
+      nil ->
+        Map.put(config, :deliver_later_strategy, Bamboo.ImmediateDeliveryStrategy)
+
+      Bamboo.ImmediateDeliveryStrategy ->
+        config
+
+      _ ->
+        raise ArgumentError, """
+        BambooSMTP.TestAdapter requires that the deliver_later_strategy is
+        Bamboo.ImmediateDeliveryStrategy
+
+        Instead it got: #{inspect(config[:deliver_later_strategy])}
+
+        Please remove the deliver_later_strategy from your config options, or
+        set it to Bamboo.ImmediateDeliveryStrategy.
+        """
+    end
+  end
+
+  @doc false
+  def supports_attachments?, do: true
+end
diff --git a/mix.exs b/mix.exs
index 4a3ca8a..6478a13 100644
--- a/mix.exs
+++ b/mix.exs
@@ -6,7 +6,7 @@ defmodule BambooSmtp.Mixfile do
   def project do
     [
       app: :bamboo_smtp,
-      version: "3.0.0",
+      version: "3.1.0",
       elixir: "~> 1.7",
       source_url: @project_url,
       homepage_url: @project_url,
@@ -27,19 +27,19 @@ defmodule BambooSmtp.Mixfile do
 
   defp deps do
     [
-      {:bamboo, "~> 1.2"},
-      {:credo, "~> 1.4.0", only: [:dev, :test]},
+      {:bamboo, "~> 1.6"},
+      {:credo, "~> 1.4.1", only: [:dev, :test]},
       {:earmark, ">= 1.3.2", only: :docs},
-      {:excoveralls, "~> 0.12.0", only: :test},
+      {:excoveralls, "~> 0.13.3", only: :test},
       {:ex_doc, ex_doc_version(), only: :docs},
-      {:gen_smtp, "~> 0.15.0"},
+      {:gen_smtp, "~> 1.0.1"},
       {:inch_ex, "~> 2.0.0", only: :docs}
     ]
   end
 
   defp ex_doc_version do
     if Version.match?(System.version(), "~> 1.7") do
-      "~> 0.21.2"
+      "~> 0.23.0"
     else
       "~> 0.18.4"
     end
diff --git a/mix.lock b/mix.lock
index 5b5d399..2c6734a 100644
--- a/mix.lock
+++ b/mix.lock
@@ -1,30 +1,29 @@
 %{
-  "bamboo": {:hex, :bamboo, "1.5.0", "1926107d58adba6620450f254dfe8a3686637a291851fba125686fa8574842af", [:mix], [{:hackney, ">= 1.13.0", [hex: :hackney, repo: "hexpm", optional: false]}, {:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: true]}, {:plug, "~> 1.0", [hex: :plug, repo: "hexpm", optional: false]}], "hexpm", "d5f3d04d154e80176fd685e2531e73870d8700679f14d25a567e448abce6298d"},
+  "bamboo": {:hex, :bamboo, "1.6.0", "adfb583bef028923aae6f22deaea6667290561da1246058556ecaeb0fec5a175", [:mix], [{:hackney, ">= 1.13.0", [hex: :hackney, repo: "hexpm", optional: false]}, {:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: true]}, {:plug, "~> 1.0", [hex: :plug, repo: "hexpm", optional: false]}], "hexpm", "454e67feacbc9b6e00553ce1d2fba003c861e0035600d59b09d6159985b17f9b"},
   "bunt": {:hex, :bunt, "0.2.0", "951c6e801e8b1d2cbe58ebbd3e616a869061ddadcc4863d0a2182541acae9a38", [:mix], [], "hexpm", "7af5c7e09fe1d40f76c8e4f9dd2be7cebd83909f31fee7cd0e9eadc567da8353"},
-  "certifi": {:hex, :certifi, "2.5.1", "867ce347f7c7d78563450a18a6a28a8090331e77fa02380b4a21962a65d36ee5", [:rebar3], [{:parse_trans, "~>3.3", [hex: :parse_trans, repo: "hexpm", optional: false]}], "hexpm", "805abd97539caf89ec6d4732c91e62ba9da0cda51ac462380bbd28ee697a8c42"},
-  "credo": {:hex, :credo, "1.4.0", "92339d4cbadd1e88b5ee43d427b639b68a11071b6f73854e33638e30a0ea11f5", [:mix], [{:bunt, "~> 0.2.0", [hex: :bunt, repo: "hexpm", optional: false]}, {:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: false]}], "hexpm", "1fd3b70dce216574ce3c18bdf510b57e7c4c85c2ec9cad4bff854abaf7e58658"},
-  "dogma": {:hex, :dogma, "0.1.13", "7b6c6ad2b3ee6501eda3bd39e197dd5198be8d520d1c175c7f713803683cf27a", [:mix], [{:poison, ">= 2.0.0", [hex: :poison, optional: false]}]},
-  "earmark": {:hex, :earmark, "1.4.4", "4821b8d05cda507189d51f2caeef370cf1e18ca5d7dfb7d31e9cafe6688106a4", [:mix], [], "hexpm", "1f93aba7340574847c0f609da787f0d79efcab51b044bb6e242cae5aca9d264d"},
-  "ex_doc": {:hex, :ex_doc, "0.21.3", "857ec876b35a587c5d9148a2512e952e24c24345552259464b98bfbb883c7b42", [:mix], [{:earmark, "~> 1.4", [hex: :earmark, repo: "hexpm", optional: false]}, {:makeup_elixir, "~> 0.14", [hex: :makeup_elixir, repo: "hexpm", optional: false]}], "hexpm", "0db1ee8d1547ab4877c5b5dffc6604ef9454e189928d5ba8967d4a58a801f161"},
-  "excoveralls": {:hex, :excoveralls, "0.12.3", "2142be7cb978a3ae78385487edda6d1aff0e482ffc6123877bb7270a8ffbcfe0", [:mix], [{:hackney, "~> 1.0", [hex: :hackney, repo: "hexpm", optional: false]}, {:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: false]}], "hexpm", "568a3e616c264283f5dea5b020783ae40eef3f7ee2163f7a67cbd7b35bcadada"},
-  "exjsx": {:hex, :exjsx, "4.0.0", "60548841e0212df401e38e63c0078ec57b33e7ea49b032c796ccad8cde794b5c", [:mix], [{:jsx, "~> 2.8.0", [hex: :jsx, repo: "hexpm", optional: false]}], "hexpm"},
-  "gen_smtp": {:hex, :gen_smtp, "0.15.0", "9f51960c17769b26833b50df0b96123605a8024738b62db747fece14eb2fbfcc", [:rebar3], [], "hexpm", "29bd14a88030980849c7ed2447b8db6d6c9278a28b11a44cafe41b791205440f"},
-  "hackney": {:hex, :hackney, "1.15.2", "07e33c794f8f8964ee86cebec1a8ed88db5070e52e904b8f12209773c1036085", [:rebar3], [{:certifi, "2.5.1", [hex: :certifi, repo: "hexpm", optional: false]}, {:idna, "6.0.0", [hex: :idna, repo: "hexpm", optional: false]}, {:metrics, "1.0.1", [hex: :metrics, repo: "hexpm", optional: false]}, {:mimerl, "~>1.1", [hex: :mimerl, repo: "hexpm", optional: false]}, {:ssl_verify_fun, "1.1.5", [hex: :ssl_verify_fun, repo: "hexpm", optional: false]}], "hexpm", "e0100f8ef7d1124222c11ad362c857d3df7cb5f4204054f9f0f4a728666591fc"},
-  "httpoison": {:hex, :httpoison, "0.9.0", "68187a2daddfabbe7ca8f7d75ef227f89f0e1507f7eecb67e4536b3c516faddb", [:mix], [{:hackney, "~> 1.6.0", [hex: :hackney, optional: false]}]},
-  "idna": {:hex, :idna, "6.0.0", "689c46cbcdf3524c44d5f3dde8001f364cd7608a99556d8fbd8239a5798d4c10", [:rebar3], [{:unicode_util_compat, "0.4.1", [hex: :unicode_util_compat, repo: "hexpm", optional: false]}], "hexpm", "4bdd305eb64e18b0273864920695cb18d7a2021f31a11b9c5fbcd9a253f936e2"},
+  "certifi": {:hex, :certifi, "2.5.2", "b7cfeae9d2ed395695dd8201c57a2d019c0c43ecaf8b8bcb9320b40d6662f340", [:rebar3], [{:parse_trans, "~>3.3", [hex: :parse_trans, repo: "hexpm", optional: false]}], "hexpm", "3b3b5f36493004ac3455966991eaf6e768ce9884693d9968055aeeeb1e575040"},
+  "credo": {:hex, :credo, "1.4.1", "16392f1edd2cdb1de9fe4004f5ab0ae612c92e230433968eab00aafd976282fc", [:mix], [{:bunt, "~> 0.2.0", [hex: :bunt, repo: "hexpm", optional: false]}, {:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: false]}], "hexpm", "155f8a2989ad77504de5d8291fa0d41320fdcaa6a1030472e9967f285f8c7692"},
+  "earmark": {:hex, :earmark, "1.4.10", "bddce5e8ea37712a5bfb01541be8ba57d3b171d3fa4f80a0be9bcf1db417bcaf", [:mix], [{:earmark_parser, ">= 1.4.10", [hex: :earmark_parser, repo: "hexpm", optional: false]}], "hexpm", "12dbfa80810478e521d3ffb941ad9fbfcbbd7debe94e1341b4c4a1b2411c1c27"},
+  "earmark_parser": {:hex, :earmark_parser, "1.4.10", "6603d7a603b9c18d3d20db69921527f82ef09990885ed7525003c7fe7dc86c56", [:mix], [], "hexpm", "8e2d5370b732385db2c9b22215c3f59c84ac7dda7ed7e544d7c459496ae519c0"},
+  "ex_doc": {:hex, :ex_doc, "0.23.0", "a069bc9b0bf8efe323ecde8c0d62afc13d308b1fa3d228b65bca5cf8703a529d", [:mix], [{:earmark_parser, "~> 1.4.0", [hex: :earmark_parser, repo: "hexpm", optional: false]}, {:makeup_elixir, "~> 0.14", [hex: :makeup_elixir, repo: "hexpm", optional: false]}], "hexpm", "f5e2c4702468b2fd11b10d39416ddadd2fcdd173ba2a0285ebd92c39827a5a16"},
+  "excoveralls": {:hex, :excoveralls, "0.13.3", "edc5f69218f84c2bf61b3609a22ddf1cec0fbf7d1ba79e59f4c16d42ea4347ed", [:mix], [{:hackney, "~> 1.16", [hex: :hackney, repo: "hexpm", optional: false]}, {:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: false]}], "hexpm", "cc26f48d2f68666380b83d8aafda0fffc65dafcc8d8650358e0b61f6a99b1154"},
+  "gen_smtp": {:hex, :gen_smtp, "1.0.1", "cc3de2898d9136ff02dc78fc8193d4fc997824fc793f5bbbada79ccde94581be", [:rebar3], [{:hut, "1.3.0", [hex: :hut, repo: "hexpm", optional: false]}, {:ranch, "1.6.2", [hex: :ranch, repo: "hexpm", optional: false]}], "hexpm", "341a98dc4dafad9cc1e4426e4988e45b31d3f235fa637408f7579a9b598c0ab6"},
+  "hackney": {:hex, :hackney, "1.16.0", "5096ac8e823e3a441477b2d187e30dd3fff1a82991a806b2003845ce72ce2d84", [:rebar3], [{:certifi, "2.5.2", [hex: :certifi, repo: "hexpm", optional: false]}, {:idna, "6.0.1", [hex: :idna, repo: "hexpm", optional: false]}, {:metrics, "1.0.1", [hex: :metrics, repo: "hexpm", optional: false]}, {:mimerl, "~>1.1", [hex: :mimerl, repo: "hexpm", optional: false]}, {:parse_trans, "3.3.0", [hex: :parse_trans, repo: "hexpm", optional: false]}, {:ssl_verify_fun, "1.1.6", [hex: :ssl_verify_fun, repo: "hexpm", optional: false]}], "hexpm", "3bf0bebbd5d3092a3543b783bf065165fa5d3ad4b899b836810e513064134e18"},
+  "hut": {:hex, :hut, "1.3.0", "71f2f054e657c03f959cf1acc43f436ea87580696528ca2a55c8afb1b06c85e7", [:"erlang.mk", :rebar, :rebar3], [], "hexpm", "7e15d28555d8a1f2b5a3a931ec120af0753e4853a4c66053db354f35bf9ab563"},
+  "idna": {:hex, :idna, "6.0.1", "1d038fb2e7668ce41fbf681d2c45902e52b3cb9e9c77b55334353b222c2ee50c", [:rebar3], [{:unicode_util_compat, "0.5.0", [hex: :unicode_util_compat, repo: "hexpm", optional: false]}], "hexpm", "a02c8a1c4fd601215bb0b0324c8a6986749f807ce35f25449ec9e69758708122"},
   "inch_ex": {:hex, :inch_ex, "2.0.0", "24268a9284a1751f2ceda569cd978e1fa394c977c45c331bb52a405de544f4de", [:mix], [{:bunt, "~> 0.2", [hex: :bunt, repo: "hexpm", optional: false]}, {:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: false]}], "hexpm", "96d0ec5ecac8cf63142d02f16b7ab7152cf0f0f1a185a80161b758383c9399a8"},
-  "jason": {:hex, :jason, "1.2.1", "12b22825e22f468c02eb3e4b9985f3d0cb8dc40b9bd704730efa11abd2708c44", [:mix], [{:decimal, "~> 1.0", [hex: :decimal, repo: "hexpm", optional: true]}], "hexpm", "b659b8571deedf60f79c5a608e15414085fa141344e2716fbd6988a084b5f993"},
-  "jsx": {:hex, :jsx, "2.8.2", "7acc7d785b5abe8a6e9adbde926a24e481f29956dd8b4df49e3e4e7bcc92a018", [:mix, :rebar3], [], "hexpm"},
-  "makeup": {:hex, :makeup, "1.0.1", "82f332e461dc6c79dbd82fbe2a9c10d48ed07146f0a478286e590c83c52010b5", [:mix], [{:nimble_parsec, "~> 0.5.0", [hex: :nimble_parsec, repo: "hexpm", optional: false]}], "hexpm", "49736fe5b66a08d8575bf5321d716bac5da20c8e6b97714fec2bcd6febcfa1f8"},
-  "makeup_elixir": {:hex, :makeup_elixir, "0.14.0", "cf8b7c66ad1cff4c14679698d532f0b5d45a3968ffbcbfd590339cb57742f1ae", [:mix], [{:makeup, "~> 1.0", [hex: :makeup, repo: "hexpm", optional: false]}], "hexpm", "d4b316c7222a85bbaa2fd7c6e90e37e953257ad196dc229505137c5e505e9eff"},
+  "jason": {:hex, :jason, "1.2.2", "ba43e3f2709fd1aa1dce90aaabfd039d000469c05c56f0b8e31978e03fa39052", [:mix], [{:decimal, "~> 1.0 or ~> 2.0", [hex: :decimal, repo: "hexpm", optional: true]}], "hexpm", "18a228f5f0058ee183f29f9eae0805c6e59d61c3b006760668d8d18ff0d12179"},
+  "makeup": {:hex, :makeup, "1.0.5", "d5a830bc42c9800ce07dd97fa94669dfb93d3bf5fcf6ea7a0c67b2e0e4a7f26c", [:mix], [{:nimble_parsec, "~> 0.5 or ~> 1.0", [hex: :nimble_parsec, repo: "hexpm", optional: false]}], "hexpm", "cfa158c02d3f5c0c665d0af11512fed3fba0144cf1aadee0f2ce17747fba2ca9"},
+  "makeup_elixir": {:hex, :makeup_elixir, "0.15.0", "98312c9f0d3730fde4049985a1105da5155bfe5c11e47bdc7406d88e01e4219b", [:mix], [{:makeup, "~> 1.0", [hex: :makeup, repo: "hexpm", optional: false]}, {:nimble_parsec, "~> 1.1", [hex: :nimble_parsec, repo: "hexpm", optional: false]}], "hexpm", "75ffa34ab1056b7e24844c90bfc62aaf6f3a37a15faa76b07bc5eba27e4a8b4a"},
   "metrics": {:hex, :metrics, "1.0.1", "25f094dea2cda98213cecc3aeff09e940299d950904393b2a29d191c346a8486", [:rebar3], [], "hexpm", "69b09adddc4f74a40716ae54d140f93beb0fb8978d8636eaded0c31b6f099f16"},
-  "mime": {:hex, :mime, "1.3.1", "30ce04ab3175b6ad0bdce0035cba77bba68b813d523d1aac73d9781b4d193cf8", [:mix], [], "hexpm", "6cbe761d6a0ca5a31a0931bf4c63204bceb64538e664a8ecf784a9a6f3b875f1"},
+  "mime": {:hex, :mime, "1.4.0", "5066f14944b470286146047d2f73518cf5cca82f8e4815cf35d196b58cf07c47", [:mix], [], "hexpm", "75fa42c4228ea9a23f70f123c74ba7cece6a03b1fd474fe13f6a7a85c6ea4ff6"},
   "mimerl": {:hex, :mimerl, "1.2.0", "67e2d3f571088d5cfd3e550c383094b47159f3eee8ffa08e64106cdf5e981be3", [:rebar3], [], "hexpm", "f278585650aa581986264638ebf698f8bb19df297f66ad91b18910dfc6e19323"},
-  "nimble_parsec": {:hex, :nimble_parsec, "0.5.3", "def21c10a9ed70ce22754fdeea0810dafd53c2db3219a0cd54cf5526377af1c6", [:mix], [], "hexpm", "589b5af56f4afca65217a1f3eb3fee7e79b09c40c742fddc1c312b3ac0b3399f"},
+  "nimble_parsec": {:hex, :nimble_parsec, "1.1.0", "3a6fca1550363552e54c216debb6a9e95bd8d32348938e13de5eda962c0d7f89", [:mix], [], "hexpm", "08eb32d66b706e913ff748f11694b17981c0b04a33ef470e33e11b3d3ac8f54b"},
   "parse_trans": {:hex, :parse_trans, "3.3.0", "09765507a3c7590a784615cfd421d101aec25098d50b89d7aa1d66646bc571c1", [:rebar3], [], "hexpm", "17ef63abde837ad30680ea7f857dd9e7ced9476cdd7b0394432af4bfc241b960"},
-  "plug": {:hex, :plug, "1.10.0", "6508295cbeb4c654860845fb95260737e4a8838d34d115ad76cd487584e2fc4d", [:mix], [{:mime, "~> 1.0", [hex: :mime, repo: "hexpm", optional: false]}, {:plug_crypto, "~> 1.1.1 or ~> 1.2", [hex: :plug_crypto, repo: "hexpm", optional: false]}, {:telemetry, "~> 0.4", [hex: :telemetry, repo: "hexpm", optional: true]}], "hexpm", "422a9727e667be1bf5ab1de03be6fa0ad67b775b2d84ed908f3264415ef29d4a"},
-  "plug_crypto": {:hex, :plug_crypto, "1.1.2", "bdd187572cc26dbd95b87136290425f2b580a116d3fb1f564216918c9730d227", [:mix], [], "hexpm", "6b8b608f895b6ffcfad49c37c7883e8df98ae19c6a28113b02aa1e9c5b22d6b5"},
-  "poison": {:hex, :poison, "3.1.0", "d9eb636610e096f86f25d9a46f35a9facac35609a7591b3be3326e99a0484665", [:mix], [], "hexpm"},
-  "ssl_verify_fun": {:hex, :ssl_verify_fun, "1.1.5", "6eaf7ad16cb568bb01753dbbd7a95ff8b91c7979482b95f38443fe2c8852a79b", [:make, :mix, :rebar3], [], "hexpm", "13104d7897e38ed7f044c4de953a6c28597d1c952075eb2e328bc6d6f2bfc496"},
-  "unicode_util_compat": {:hex, :unicode_util_compat, "0.4.1", "d869e4c68901dd9531385bb0c8c40444ebf624e60b6962d95952775cac5e90cd", [:rebar3], [], "hexpm", "1d1848c40487cdb0b30e8ed975e34e025860c02e419cb615d255849f3427439d"},
+  "plug": {:hex, :plug, "1.11.0", "f17217525597628298998bc3baed9f8ea1fa3f1160aa9871aee6df47a6e4d38e", [:mix], [{:mime, "~> 1.0", [hex: :mime, repo: "hexpm", optional: false]}, {:plug_crypto, "~> 1.1.1 or ~> 1.2", [hex: :plug_crypto, repo: "hexpm", optional: false]}, {:telemetry, "~> 0.4", [hex: :telemetry, repo: "hexpm", optional: false]}], "hexpm", "2d9c633f0499f9dc5c2fd069161af4e2e7756890b81adcbb2ceaa074e8308876"},
+  "plug_crypto": {:hex, :plug_crypto, "1.2.0", "1cb20793aa63a6c619dd18bb33d7a3aa94818e5fd39ad357051a67f26dfa2df6", [:mix], [], "hexpm", "a48b538ae8bf381ffac344520755f3007cc10bd8e90b240af98ea29b69683fc2"},
+  "ranch": {:hex, :ranch, "1.6.2", "6db93c78f411ee033dbb18ba8234c5574883acb9a75af0fb90a9b82ea46afa00", [:rebar3], [], "hexpm", "0b65d1de7f1ebe96feb57803c8d698f7776b663ba10b6dcff34527e4a4f8ff43"},
+  "ssl_verify_fun": {:hex, :ssl_verify_fun, "1.1.6", "cf344f5692c82d2cd7554f5ec8fd961548d4fd09e7d22f5b62482e5aeaebd4b0", [:make, :mix, :rebar3], [], "hexpm", "bdb0d2471f453c88ff3908e7686f86f9be327d065cc1ec16fa4540197ea04680"},
+  "telemetry": {:hex, :telemetry, "0.4.2", "2808c992455e08d6177322f14d3bdb6b625fbcfd233a73505870d8738a2f4599", [:rebar3], [], "hexpm", "2d1419bd9dda6a206d7b5852179511722e2b18812310d304620c7bd92a13fcef"},
+  "unicode_util_compat": {:hex, :unicode_util_compat, "0.5.0", "8516502659002cec19e244ebd90d312183064be95025a319a6c7e89f4bccd65b", [:rebar3], [], "hexpm", "d48d002e15f5cc105a696cf2f1bbb3fc72b4b770a184d8420c8db20da2674b38"},
 }
diff --git a/test/attachments/attachment_one.txt b/test/attachments/attachment_one.txt
new file mode 100644
index 0000000..3e2caaf
--- /dev/null
+++ b/test/attachments/attachment_one.txt
@@ -0,0 +1 @@
+Attachment One.
diff --git a/test/attachments/attachment_two.txt b/test/attachments/attachment_two.txt
new file mode 100644
index 0000000..e4888d3
--- /dev/null
+++ b/test/attachments/attachment_two.txt
@@ -0,0 +1 @@
+Attachment Two.
diff --git a/test/lib/bamboo/adapters/smtp_adapter_test.exs b/test/lib/bamboo/adapters/smtp_adapter_test.exs
index 3d70020..08b9ac9 100644
--- a/test/lib/bamboo/adapters/smtp_adapter_test.exs
+++ b/test/lib/bamboo/adapters/smtp_adapter_test.exs
@@ -12,7 +12,7 @@ defmodule Bamboo.SMTPAdapterTest do
       {:ok, args}
     end
 
-    def start_link do
+    def start_link(_) do
       GenServer.start_link(__MODULE__, [], name: __MODULE__)
     end
 
@@ -115,7 +115,7 @@ defmodule Bamboo.SMTPAdapterTest do
   ]
 
   setup do
-    FakeGenSMTP.start_link()
+    start_supervised!(FakeGenSMTP)
 
     :ok
   end
@@ -434,7 +434,7 @@ defmodule Bamboo.SMTPAdapterTest do
     assert format_email_as_string(bamboo_email.from, false) == from
     assert format_email(bamboo_email.to ++ bamboo_email.cc ++ bamboo_email.bcc, false) == to
 
-    rfc822_subject = "Subject: =?UTF-8?B?SGVsbG8gZnJvbSBCYW1ib28=?=\r\n"
+    rfc822_subject = "Subject: =?UTF-8?B?Hello from Bamboo?=\r\n"
     assert String.contains?(raw_email, rfc822_subject)
 
     assert String.contains?(raw_email, "From: #{format_email_as_string(bamboo_email.from)}\r\n")
@@ -499,7 +499,7 @@ defmodule Bamboo.SMTPAdapterTest do
     assert format_email_as_string(bamboo_email.from, false) == from
     assert format_email(bamboo_email.to ++ bamboo_email.cc ++ bamboo_email.bcc, false) == to
 
-    rfc822_subject = "Subject: =?UTF-8?B?SGVsbG8gZnJvbSBCYW1ib28=?=\r\n"
+    rfc822_subject = "Subject: =?UTF-8?B?Hello from Bamboo?=\r\n"
     assert String.contains?(raw_email, rfc822_subject)
 
     assert String.contains?(raw_email, "From: #{format_email_as_string(bamboo_email.from)}\r\n")
@@ -547,7 +547,7 @@ defmodule Bamboo.SMTPAdapterTest do
     assert format_email_as_string(bamboo_email.from, false) == from
     assert format_email(bamboo_email.to ++ bamboo_email.cc ++ bamboo_email.bcc, false) == to
 
-    rfc822_subject = "Subject: =?UTF-8?B?SGVsbG8gZnJvbSBCYW1ib28=?=\r\n"
+    rfc822_subject = "Subject: =?UTF-8?B?Hello from Bamboo?=\r\n"
     assert String.contains?(raw_email, rfc822_subject)
 
     assert String.contains?(raw_email, "From: #{format_email_as_string(bamboo_email.from)}\r\n")
@@ -597,7 +597,7 @@ defmodule Bamboo.SMTPAdapterTest do
     assert format_email_as_string(bamboo_email.from, false) == from
     assert format_email(bamboo_email.to ++ bamboo_email.cc ++ bamboo_email.bcc, false) == to
 
-    rfc822_subject = "Subject: =?UTF-8?B?SGVsbG8gZnJvbSBCYW1ib28=?=\r\n"
+    rfc822_subject = "Subject: =?UTF-8?B?Hello from Bamboo?=\r\n"
     assert String.contains?(raw_email, rfc822_subject)
 
     assert String.contains?(raw_email, "From: #{format_email_as_string(bamboo_email.from)}\r\n")
@@ -646,7 +646,7 @@ defmodule Bamboo.SMTPAdapterTest do
     assert format_email_as_string(bamboo_email.from, false) == from
     assert format_email(bamboo_email.to ++ bamboo_email.cc ++ bamboo_email.bcc, false) == to
 
-    rfc822_subject = "Subject: =?UTF-8?B?SGVsbG8gZnJvbSBCYW1ib28=?=\r\n"
+    rfc822_subject = "Subject: =?UTF-8?B?Hello from Bamboo?=\r\n"
     assert String.contains?(raw_email, rfc822_subject)
 
     assert String.contains?(raw_email, "From: #{format_email_as_string(bamboo_email.from)}\r\n")
@@ -675,6 +675,83 @@ defmodule Bamboo.SMTPAdapterTest do
     assert_configuration(bamboo_config, gen_smtp_config)
   end
 
+  test "email looks fine when they have non-ASCII characters in subject, from and to" do
+    bamboo_email =
+      new_email(
+        from: {"Awesome Person 😎", "awesome@person.local"},
+        to: {"Person Awesome 🤩", "person@awesome.local"},
+        subject: "Hello! 👋"
+      )
+
+    bamboo_config = configuration()
+
+    {:ok, "200 Ok 1234567890"} = SMTPAdapter.deliver(bamboo_email, bamboo_config)
+
+    assert 1 = length(FakeGenSMTP.fetch_sent_emails())
+
+    [{{from, to, raw_email}, gen_smtp_config}] = FakeGenSMTP.fetch_sent_emails()
+
+    [multipart_header] =
+      Regex.run(
+        ~r{Content-Type: multipart/alternative; boundary="([^"]+)"\r\n},
+        raw_email,
+        capture: :all_but_first
+      )
+
+    assert format_email_as_string(bamboo_email.from, false) == from
+    assert format_email(bamboo_email.to ++ bamboo_email.cc ++ bamboo_email.bcc, false) == to
+
+    rfc822_subject = "Subject: =?UTF-8?B?SGVsbG8hIPCfkYs=?=\r\n"
+    assert String.contains?(raw_email, rfc822_subject)
+
+    assert String.contains?(raw_email, "From: #{format_email_as_string(bamboo_email.from)}\r\n")
+    assert String.contains?(raw_email, "To: #{format_email_as_string(bamboo_email.to)}\r\n")
+    assert String.contains?(raw_email, "Cc: #{format_email_as_string(bamboo_email.cc)}\r\n")
+    assert String.contains?(raw_email, "Bcc: #{format_email_as_string(bamboo_email.bcc)}\r\n")
+    assert String.contains?(raw_email, "Reply-To: reply@doe.com\r\n")
+    assert String.contains?(raw_email, "MIME-Version: 1.0\r\n")
+
+    assert String.contains?(
+             raw_email,
+             "--#{multipart_header}\r\n" <>
+               "Content-Type: text/html;charset=UTF-8\r\n" <>
+               "Content-Transfer-Encoding: base64\r\n" <>
+               "\r\n" <>
+               "#{SMTPAdapter.base64_and_split(bamboo_email.html_body)}\r\n"
+           )
+
+    assert String.contains?(
+             raw_email,
+             "--#{multipart_header}\r\n" <>
+               "Content-Type: text/plain;charset=UTF-8\r\n" <>
+               "\r\n"
+           )
+
+    assert_configuration(bamboo_config, gen_smtp_config)
+  end
+
+  test "email have a Content-ID properly set when attaching files with content_id" do
+    bamboo_email =
+      new_email()
+      |> Bamboo.Email.put_attachment(Path.absname("test/attachments/attachment_one.txt"),
+        content_id: "12345"
+      )
+      |> Bamboo.Email.put_attachment(Path.absname("test/attachments/attachment_two.txt"),
+        content_id: "54321"
+      )
+
+    bamboo_config = configuration()
+
+    {:ok, "200 Ok 1234567890"} = SMTPAdapter.deliver(bamboo_email, bamboo_config)
+
+    assert 1 = length(FakeGenSMTP.fetch_sent_emails())
+
+    [{{_from, _to, raw_email}, _gen_smtp_config}] = FakeGenSMTP.fetch_sent_emails()
+
+    assert Regex.run(~r{Content-ID: <12345>\r\n}, raw_email, capture: :all_but_first)
+    assert Regex.run(~r{Content-ID: <54321>\r\n}, raw_email, capture: :all_but_first)
+  end
+
   test "check rfc822 encoding for subject" do
     bamboo_email =
       @email_in_utf8
@@ -709,7 +786,17 @@ defmodule Bamboo.SMTPAdapterTest do
   end
 
   defp rfc822_encode(content) do
-    "=?UTF-8?B?#{Base.encode64(content)}?="
+    if contains_only_ascii_characters?(content) do
+      "=?UTF-8?B?#{content}?="
+    else
+      "=?UTF-8?B?#{Base.encode64(content)}?="
+    end
+  end
+
+  defp contains_only_ascii_characters?(content) do
+    content
+    |> String.to_charlist()
+    |> List.ascii_printable?()
   end
 
   defp assert_configuration(bamboo_config, gen_smtp_config) do