Are there any SMTP servers that deliver mail synchronously? So that if the delivery command fails, the sender gets an SMTP error? This would allow returning error messages without risk of backscatter.
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I’d previously had to switch from bouncing such messages to just dropping them, due to backscatter spam. But messages being ignored is confusing to legitimate users. After my last tweet, I ended up hacking something up… it’s ugly though.
You should be able to check_sender_access against a MySQL table or something in that case.