Simple php proxy script to validate a token
Dovecot config, allows an introspection, but configuring the fields in theproper headers etc is somewhat tricky. It also allows a simple post of the token, so its easier and quicker to handle it in this way.
More details why and how you can find here: https://color-domain.com/projects/kinde/ I've done a write up here on configuring Sendmail && Dovecot && Roundcube with Kinde.com oAuth2.