Thank you for the advice.
I had already tried this "method" though. lol
I asked him to show me the emails or even tell me what was said. He said they were deleted and that they "never really talked about anything."
With the exception of one, I was told they were sent from his work account.
I stumbled upon the last email by mistake, checking to see if a file I sent
from his laptop to my pc was still in the "sent" folder there. That's how I found out about the situation.

I continued to question him though (my bad) and was told that my mother had told him to go out and "have fun" by himself that night/weekend. He was trying to pass the blame onto her in a way. And, that the female in question tracked him down, by finding his employers' website (the school board) and finding his email address from the site's directory.
He also told me that in one email she had said she told her fiance
she had been talking to him, and he said it was okay as long as they weren't having lunch dates, or more. I also asked him if he told her that he was married, he wouldn't give me an answer for that, either.

After a long night of arguing, I just wanted to try to move on and forget things. Knowing the type of personality I have, this won't be easy.
I'd like to think, if something were going on, forget it. I guess it evens out for all the times I let him down with my mood swings/disorders.

Thanks everyone for your input!