This change has a tradeoff.
It increases our backscatter vulnerability - the subject could have spammy content - but it's extremely valuable to the user to know exactly which message was rejected.
If you sent two at the same time, and only one was rejected, you would have no way of knowing which worked and which to resend without going to the website (which is what email-in is trying to avoid, kinda).
- Replace implicit return code-system in Email::Receiver with proper exception system
- Update tests to check for exceptions instead
- Test the PollMailbox for expected failures
- Add proper email-handling of problematic emails
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