MailHop And Postini

Our MailHop Relay and MailHop Backup MX services are intended for primary mail servers which you personally administer. It is strongly recommended that you do not use third-party mail hosting or spam scanning services with these MailHop services. Many providers do not allow customers to whitelist particular sending servers; as a result, the providers' anti-spam measures often accidentally blacklist our servers, resulting in lost mail.

A customer wishes to use Postini for spam scanning, but their ISP blocks port 25. The answer would seem simple: List Postini's mail servers in the MX records for the domain, and use the MailHop Relay server as the destination mail server in Postini. Logically, mail would be received by Postini, scanned, redelivered to Relay, and finally redelivered to their server on the alternate port.

However, MailHop Relay is designed to forward mail to the highest-priority mail server listed in a domain's MX records (if MailHop Relay is not the primary). Furthermore, MailHop Relay and Postini both use a feature known as a call-ahead to verify the existence of an email address before it will accept the full message. Due to this unique combination of behaviors, the following loop occurs:

• A sending mail server connects to Postini.
• Postini receives the connection, reviews the address, and looks up the destination server, MailHop Relay.
• Postini performs a call-ahead to MailHop Relay to see if the address exists.
• MailHop Relay receives the connection, reviews the address, and looks up the MX records for the domain. MailHop Relay sees Postini as the highest-priority mail server.
• MailHop Relay performs a call-ahead to Postini to see if the address exists.
• Postini receives the connection, reviews the address, and looks up the destination server, MailHop Relay.
• Postini performs a call-ahead...

This permanent loop of call-aheads is not detected by either side, as each one is considered to be a new connection, and the mail is never delivered.