XCON: You Cannot Send SMTP Mail Through a Firewall (305007)

SYMPTOMS

If you send a message to a Simple Mail Transfer Protocol (SMTP) recipient over the Internet Mail Service by using Microsoft Outlook Express or any other Post Office Protocol version 3 (POP3) or Internet Message Access Protocol Version 4rev1 (IMAP4) client, you may receive a non-delivery report (NDR) that is similar to:

The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was 'XXX@XXX.XXX'. Subject 'XXXX', Account: 'XXX', Server: 'XXX.XXX.XXX.XXX', Protocol: SMTP, Server Response:
'550 Requested action not taken: mailbox unavailable', Port: 25, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC79

CAUSE

This issue may occur if:

Rerouting is selected on the Routing tab of the Internet Mail Service. -and-
The Hosts and clients that successfully authenticate routing restriction is selected. -and-
A Watchguard Firebox firewall is being used.

The Watchguard Firebox firewall can be configured to strip SMTP commands. To use authentication, the following command verbs must be turned on:

Auth
Auth-logon

If the authentication headers are denied on the firewall, the mail flow is stopped externally.

RESOLUTION

To resolve this issue, configure the firewall to allow all of the Exchange Server SMTP command verbs to pass through. If you have to, turn off the headers that are being stripped one by one.

MORE INFORMATION

Determine whether or not you can successfully send mail from a POP3 or IMAP4 client if you connect directly to the Exchange Server computer, instead of connecting through the firewall.