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.
I'm not sure about *delivering* synchronously, but you can perform checks synchronously (e.g. reject nonexistent addresses or mails that fail some spam checker). What condition are you looking to error out on specifically? Postfix can probably do what you want.
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See e.g. http://www.postfix.org/ADDRESS_VERIFICATION_README.html#how … which uses probe messages to verify both local and remote addresses.
Thanks. Twitter will use this to make your timeline better. UndoUndo
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Trying to reject mail from non-subscribers to a certain mailing list (Mailman 2). (The non-subscriber policy itself has been there for a long time, and isn’t much of an etiquette issue in this context.)
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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.
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