I have the same problem, tried to modify the URL to https (and rebuild app) but still having that problem:
<22>Apr 13 10:18:10 postfix/smtpd[323]: connect from mail-vs1-f50.google.com[209.85.217.50]
<22>Apr 13 10:18:11 postfix/smtpd[323]: 403AF81D15: client=mail-vs1-f50.google.com[209.85.217.50]
<22>Apr 13 10:18:11 postfix/cleanup[329]: 403AF81D15: message-id=<XXXXXXXXX@mail.gmail.com>
<22>Apr 13 10:18:11 postfix/qmgr[80]: 403AF81D15: from=<XXXXXXXXX@gmail.com>, size=2555, nrcpt=1 (queue active)
<22>Apr 13 10:18:11 postfix/smtpd[323]: disconnect from mail-vs1-f50.google.com[209.85.217.50] ehlo=1 mail=1 rcpt=1 data=1 quit=1 commands=5
<23>Apr 13 10:18:11 receive-mail[331]: Recipient: testing@mydomain.com
<19>Apr 13 10:18:11 receive-mail[331]: Failed to POST the e-mail to http://forum.mydomain.com/admin/email/handle_mail: 301
<22>Apr 13 10:18:11 postfix/pipe[330]: 403AF81D15: to=<testing@mydomain.com.ar>, relay=discourse, delay=0.72, delays=0.55/0.01/0/0.16, dsn=4.3.0, status=deferred (temporary failure)
Maybe I need to enable SSL options in e-mail template? My discourse it’s behind a global nginx instance (I see port 25 exposed in mail-reciever.template).
Thanks for your time, as always.