Sure... but unless I'm misled, with UDP there simply is no way to know for sure excepting an out of band response—a visit to a tokenized URI sent in a message or a return message from the same domain/address. So no checks will be guaranteed. "Heroic" messages to circumvent misused standards / client-fail seems a mistake.

In reply to Re^3: Determine if domain is actually used for email by Your Mother
in thread Determine if domain is actually used for email by vit

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