Sorry to hear that but step back a minute! You would believe anything coming from the mouth of an adulterer? I wouldn't because the man has no morals.
Listen, you've probably known all along that this was the case deep down inside but were willing to at least attempt to work on it.
If this is a deal breaker for you then so be it. But think, could it be that he sent it because he's losing his grip and WANTS you to get pissed off so that she can run to him for consolment? Put that in your pipe and smoke on it for awhile.