Exchange doesn’t recognize it as an email, since it doesn’t support/understand wildcards.
The workaround I found on technet was to add a “Catch-All” email address to your transport server, that just sends everything not to a valid exchange email to a catch email.
Multiple challenges with that obviously. Emails that aren’t actually to the Discourse reply email would clog up the system, plus you could only have 1 email like that for your whole exchange organization. So, not the best workaround.
We will probably just end up setting up a gmail account for it. Drawback of that is data flowing into and out of our enterprise for a forum that is internal/intranet only. And, someone will have to periodically check it for spam filter catching the emails, or other weirdness.
thanks for the comments and questions though.