Well, what a puzzle. I will have to investigate. I also tried it with Gmail, and the failure was definitely between Postfix and Dovecot LMTP, with Dovecot failing to advertise SMTPUTF8 support. My setup is much more minimal, I use file-based virtual mailboxes and aliases, and only OpenDKIM and SpamAssassin milters.
Thank you for your reply, André!
