How address verification works

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.

Internet	->	Postfix SMTP server	<->		Postfix verify server			<->		Address verification database
				| probe messages v			^ delivery status |
				Postfix queue		->	Postfix delivery agents

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.