Could be any multitude of things that are really out of the scope of the UBB.
firstly, the SMTP server would be on your new server, and the A record would have to be set to the new server from the old.
A quick fix would be to try to just not use an SMTP server and see if it sends mails with php's default mailer.
Other than that you would have to test the smtp server to be sure it's configured and setup to allow mail relays
