Address verification is a feature that allows the Postfix SMTP
server to block a sender (MAIL FROM) or recipient (RCPT TO) address
until the address has been verified to be deliverable.
The technique has obvious uses to reject junk mail
with an unreplyable sender address.
The technique may also be useful to block mail for undeliverable
recipients, for example on a mail
relay host that does not have a
list of all the valid recipient addresses. This prevents undeliverable
junk mail from entering the queue, so that Postfix doesn't have to
waste resources trying to send MAILER-DAEMON messages back.
This feature is available in Postfix version 2.1 and later.
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