What’s the To address on these e-mails? If Discourse is rejecting them, then it’s presumably an invalid recipient, in which case the fast-rejection code might need to be tweaked a little to catch whatever it is you’re seeing. That’ll stop Discourse sending a rejection back to itself, because it’ll never even see the message.
Discourse shouldn’t be rejecting the rejection e-mail, though, because the initial rejection should be marked as auto-generated, and we shouldn’t be responding to auto-generated mail. Presumably one of those "should"s isn’t true, which is a bug.
Otherwise, it’s reasonable that Discourse ignore e-mail sent to it from its own address; I can’t think of a legitimate circumstance in which that would happen.
Finally, it’s not a particularly good idea to disable rejections entirely for messages sent to mangled variations of the reply-to address; e-mail clients (and servers) are universally terrible, and do all manner of unspeakable things. Having a response back saying “everything is awful” is more user-friendly (in general) than blackholing mail.