In Sendmail version 8.12, MX records will be looked up in DNS for FallBackMXhost. Since, Yahoo has a bogus MX record (nomail.yahoo.com) for all of its mailserver names, including smtp.sbcglobal.yahoo.com, this would cause the FallBack MX host to bounce email if Sendmail ever forwarded to it.
To use the old behavior (no MX lookups), you have to put the fallback server's domain name in square brackets in your .mc configuration file and run it through m4 again, copy it in place of your current config and restart sendmail.
Below is the line you need to add or replace in your sndmail.mc file. If you use a different FallBack MX host, then just change the domain name inside the square brackets to the server you use.
by rolande edited by kmac1
last modified: 2003-12-18 12:59:44