The mails sent by the forum service are sent without a To header. While those sort of mails are valid, Amazon SES rejects them.
Amazon SES is likely not the only service having issue with those sort of mails (and anyway it might affects a non negligible amount of users given the market share of Amazon) so we should try to work around it.

One easy way to fix it is probably to do the same thing than the massmail feature and to set `sys_noreply` in the To header. The change should be done in a way that affects all the services so it does not have to be fixed locally each time.


You will see the following entry in the Postfix logs when the issue occurs:
554 Transaction failed: Empty required header 'To'. (in reply to end of DATA command))