An IMAP/POP/SMTP proxy that transparently adds OAuth 2.0 authentication for email clients that don't support this method. - GitHub - simonrob/email-oauth2-proxy: An IMAP/POP/SMTP proxy that transparently adds OAuth 2.0 authentication for email clients that don't support this method.

Also has a plugin that can rewrite IMAP commands.

rspamd inbound spam filters for MXroute.

(old) collection of sendmail m4 hacks and features

FairEmail - Open source, privacy friendly email app for Android