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Dockerized SMTP relay

Docker Automated build

What?

SMTP relay with local queue.

Why?

Sending emails through a controlled network (possibly localhost or LAN) is faster and more controlled than sending them directly through the Internet.

This container retains email queue in a volume under /var/spool/postfix, so in case your network fails or your real SMTP server is down for maintenance or whatever, queue will be sent when network connection is restored.

This way your app can send emails faster, forget about possible temporary network failures, and concentrate on its business.

How?

Configure through these environment variables:

  • MAILNAME: The default host for cron job mails.
  • MAIL_RELAY_HOST: The real SMTP server (e.g. smtp.mailgun.org).
  • MAIL_RELAY_PORT: The port in MAIL_RELAY_HOST. Depending on the port, a specific security configuration will be used.
  • MAIL_RELAY_USER: The user to authenticate in MAIL_RELAY_HOST.
  • MAIL_RELAY_PASS: The password to authenticate in MAIL_RELAY_HOST.
  • MAIL_CANONICAL_DOMAINS: A space-separated list of domains that are considered canonical.
  • MAIL_NON_CANONICAL_DEFAULT: A domain that should be found in the list of MAIL_CANONICAL_DOMAINS, which will be used as the replacement domain when a non-canonical message comes in. Leave it empty to skip that replacement system.
  • MAIL_CANONICAL_PREFIX: Defaults to noreply+, and it is what will be prefixed to replaced non-canonical sender addresses.
  • MESSAGE_SIZE_LIMIT in bytes, defaults to 50MiB. Most generous servers offer a limit of 25iMB (Gmail, Mailgun...), so by defaulting to 50MiB, basically we are forcing the remote server to fail in case of a big email, instead of making the local relay to fail. Change at will if you prefer a different behavior.
  • ROUTE_CUSTOM space separated list of subnets in the CIDR standard notation (e.g 192.168.0.0/16).

Examples

SMTP relay via Gmail

docker container run \
    -e MAIL_RELAY_HOST='smtp.gmail.com' \
    -e MAIL_RELAY_PORT='587' \
    -e MAIL_RELAY_USER='[email protected]' \
    -e MAIL_RELAY_PASS='your_gmail_pass' \
    tecnativa/postfix-relay

FAQ

What Is A Canonical Domain

It means "domains that are allowed to send from here".

Suppose your app allows users to define their own emails, and that one is used to send emails to other users from the system.

If you only own the example.com and example.net domains, but somebody configures his email as [email protected]. If you send this email as it came, SPAM filters will block it.

By defining MAIL_CANONICAL_DOMAINS=example.com example.net and MAIL_NON_CANONICAL_DEFAULT=example.com, the mail would be modified as if it came from [email protected], and SPAM filters will be happy with that.