6.22. Why do virtual host domains (CNAME aliases) get replaced by the primary host domain in outgoing mail?

Assuming that the host_name attribute on the list's General Options page is the correct virtual host name, the most likely cause of this is that the replacement is done by the outgoing MTA because the virtual host name is a CNAME alias for the primary host name.

The RFCs aren't completely clear on all aspects of this, but at least RFC 1123 (STD 3) is clear that domains in envelope addresses must not be CNAMEs. They must be names with an A record or an MX record that resolves to an A record. See the thread at http://mail.python.org/pipermail/mailman-users/2007-April/056483.html for more on this.

In any case, various MTA's will rewrite the CNAME alias as the canonical host name. For this and other reasons, any host name that sends mail must not be a CNAME. It must have a DNS A record or at least one MX record. Additionally, target host names in MX records must have A records.

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This is one of many Frequently Asked Questions.