I’ve also tried setting the reply by email address settings in discourse to discourse.xxx+%{reply_to}@gmail.com but then when the email gets delivered, gmail seems to think that the domain yyy.com (discourse SMTP domain) is trying to spoof the gmail domain and it ends up marking it as spam. It appears that setting a reply to domain different from the senders domain is triggering a DMARC and SPF failure.
ARC-Authentication-Results: i=1; mx.google.com;
spf=softfail (google.com: domain of transitioning discussion.xxx+verp-b9c40db917ca04993dd3433cc9748518@gmail.com does not designate y.y.y.y as permitted sender) smtp.mailfrom=discussion.xxx+verp-b9c40db917ca04993dd3433cc9748518@gmail.com;
dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=yyy.com
Return-Path: <discussion.xxx+verp-b9c40db917ca04993dd3433cc9748518@gmail.com>