<A target="_blank" href="http://dominothoughts.com/domthoughts/domthoughts.nsf/dx/smtp-mail-tries-to-route-to-a-record-instead-of-mx.htm">Ted Hardenburgh has found that, sometimes</A>, a correctly addressed message will be rejected with an "SMTP Permanent Error" message because the MX record isn't correct. He gives more information on the problem and a way to fix it.