A sender or recipient address is verified by probing the nearest
MTA for that address, without actually delivering mail. The nearest
MTA could be Postfix itself, or it could be a remote MTA (SMTP
interruptus). Probe messages are like normal mail, except that
they are never delivered, deferred or bounced; probe messages are
always discarded.
With Postfix address verification turned on, normal mail will
suffer only a short delay of up to 6 seconds while an address is
being verified for the first time. Once an address status is known,
the status is cached and Postfix replies immediately.
When verification takes too long the Postfix SMTP server defers
the sender or recipient address with a 450 reply. Normal mail
clients will connect again after some delay. The address verification
delay is configurable with the
main.cf
address_verify_poll_count
and
address_verify_poll_delay parameters. See
postconf(5) for
details.
