A better message would be good in that case.
I think you mean that if Discourse only finds out about the bounce from a Mailgun webhook then the details of the bounce never get sent to Discourse. Is that right?
If it is right – would there be any way for those details to be sent to Discourse and recorded and displayed in the same place as bounces received by Discourse? I think the Mailgun API would allow that ("Discourse::NotFound" error when click "Email Type" field on admin/email/bounced - #8 by Jonathan5).
In what circumstance would Discourse handle bounces rather than the external email service? (I know Discourse can bounce/reject emails, but I mean handling bounce messages received from intended email recipients.)