4.20. Who should deal with transient DNS errors?
Sometimes, Mailman tries to send mail to domains which exist but do not have a MX (mail exchanger) or an A (address) record. This may happen for example when a spam comes from a newly reserved domain, who has not been setup to receive mail.
Most MTA are configured to reject mail for such a domain with a temporary failure exit code (such as 450), because the absence of those records may be caused by a transient network outage. If your local MTA has been configured this way, it will reject mail from Mailman with this temporary failure exit code, and Mailman will try to resent the mail every minute. Considering that the DNS lookup may easily take up to 30 seconds in case of a network problem, this may slow down Mailman mail delivery by a huge factor.
One solution is to let your MTA deal with this situation instead of Mailman. Configure your MTA so that it always accepts mail coming from Mailman. In Postfix for example, this is done by using a "client_access" restriction (allowing mail from localhost if Mailman is running on the same machine as Postfix) before the "reject_unknown_recipient_domain" restriction.
Converted from the Mailman FAQ Wizard
This is one of many Frequently Asked Questions.