If a message has the
header field, the SMTP and Local Delivery modules do not send the message to its recipients.
The SMTP module connects to all hosts the message is addressed to, then the module sends all recipient addresses to those hosts, but it does not send the message itself.
The Local Delivery module checks if the account exists, but the module does not try to apply the Account Rules to the message and the module does not store the message in the Account Inbox.
This feature can be used to verify addresses on large mailing lists: if an address contains an unknown domain name, or the host is not unreachable, or if the host rejects a user address, an error message is generated in the regular way, and can be used to detect "bad" addresses and to "clean" the mailing list.