If a mailserver is configured on your webhost locally but your DNS zonefile correctly reflects that your external mailserver is elsewhere then all normal mail will arrive correctly at the external mailserver. On Linux PHP (and probably PERL and other script languages) connects (by default) to the local sendmail daemon. Sendmail only checks if the local mailserver is receiving mail for the destination host. It doesn't check the MX (DNS) record to see if that behaviour is correct and will silently accept the mail. So PHP mail() function reports a mail success. At (mt) you'll need to disable your mailserver for that domain. See this knowledge base article. If you're hosting DNS externally too then you should disable (or Slave) your DNS for that domain as well.