I just fixed the inbound email on my hosted forum. I tested and a failed email comes FROM a different address than normal forum emails.
in our case, good message emails come from mail+[key]@domain but failures come from support@domain. That in and of themselves is enough to make it clear the email is from the forum.
The underlying question is - should - failure emails be setup to get swept into the same actions as normal emails? If they should remain in someone’s inbox, then no change is really needed.
If they should be routed along with the rest of the forum email, then having the reply titles respect the subject rules would be a good change.