Tenho o mesmo problema. Tentei modificar a URL para https (e reconstruir o app), mas o problema persiste:
<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)
Talvez eu precise habilitar as opções SSL no modelo de e-mail? Meu Discourse está atrás de uma instância global do nginx (vejo a porta 25 exposta no mail-reciever.template).
Obrigado pelo seu tempo, como sempre.