Oh well, my replies can be a bit off since I don’t understand much email-related things.
My reply-by-email setting is indeed a gmail address that uses “+” and stuff.
If the reply to by email setting is off, then Discourse email sending works (if I remember well).
If the reply to by email setting is on, and if I don’t verify the gmail address in SES, then Discourse won’t send any emails, whether they have a gmail reply-to address or not (for example, the test email, or a digest email don’t have the gmail reply-to address), and throw a 554 Message rejected: Email address is not verified error.
If I verify the gmail address on SES, then all the emails sent by Discourse seem to work, whether they have a gmail reply-to address or not.
Of course, I’d like to know a bit more about how all these things work, but as long as it works… That’s my primary concern. 