MBF Knowledge Base

Should I use a wildcard MX for my domain?

If at all possible, no.

Wildcard MX records have lots of semantic "gotcha"s. For example, they will match a host "unknown.your.domain" -- if you don't explicitly test for unknown hosts in your domain, you will get "MX list for hostname points back to hostname" or "config error: mail loops back to myself".

See RFCs 1535, 1536, and 1912 (updates RFC 1537) for more detail and other related (or common) problems. See also _DNS and BIND_ by Albitz and Liu.

They can also cause your system to add your domain to outgoing Fully Qualified Domain Names (FQDNs) in a desperate attempt to get the mail to where it's supposed to go, but because *.your.domain is valid due to the wildcard MX, delivery to not.real.domain.your.domain will get dumped on you, and you may even find yourself in a loop as the domain keeps getting tacked on time after time after time (the "config error: mail loops back to myself" problem).

Wildcard MX records are just a bad idea, plain and simple. They don't work the way you'd expect, and virtually no one gets them right. Avoid them at all costs.