Who do i believe....?

ok, so i found out my man was writing dirty e mails to a girl who lives bout an hour an a half away... at first he lied, but when she sent me the proof he confessed. it was jus emails nothing else. well we worked thru it an he promised never to tak to her again an i believed him...well she emails me today an says they are still talking....but she cant prove it this time. he says she is lying jus to p*ss me off. who do i believe????