I'm running dovecot # 2.3.2.1 - Pigeonhole version 0.5.2 () - OS: Linux 4.18.0-12-generic x86_64 Ubuntu 18.10 with Submission.

It works great except with apple mail (Iphone).

I get an error with the MTA  (postfix):

""postfix/submission/smtpd[32552]: warning: non-SMTP command from mail.example.org[xx.xx.xx.xx]: Content-Transfer-Encoding: 7bit""

with other mail-client(s) (Outlook (Desktop and Iphone app)) i have no problem and it proxy-sends the e-mail beautiful out to the recipient.

I think it is some kind of bug or maybe it just isn't supported (yet), cause as you said it is still "pretty basic".

Best Regards