This article applies to:

• Trustwave MailMarshal (SEG) 6.X and above

Question:

• Why am I receiving 421 errors when trying to connect to MailMarshal SMTP?

Symptoms:

• 421 SMTP service not available message received when attempting to send email through MailMarshal. 

Information:

A legitimate host tries to send an email through MailMarshal SMTP but they receive the SMTP response 421, Service not Available, which is not the expected behavior if the host is allowed to relay email through MailMarshal.

The message is the expected behavior if a host has tried to send email through MailMarshal and it has triggered the DOS and DHA policies.

Hosts permitted to relay, and non-routable IP addresses, are exempted from these blocks. However, other legitimate servers might trigger these policies because they make a large number of connections to MailMarshal. You may want to exclude additional servers from DOS and DHA policies.

Resolution:

Follow the steps below to add the SMTP host to the list of exclusions:

1. In the Configurator or Management Console, navigate to Array Properties | Attack Prevention tab.
2. On the Advanced or Exclusions window, you can add one or more exclusions.
3. Enter a Single IP address or an IP address range of the host(s) that you want to allow to send through MailMarshal.