Is he cheating?

My boyfriend has several platonic girl friends that I've never minded. However, I heard from a mutual friend that he had become very close with a girl he was working with. As in they were talking for hours a day sometimes having lunch with each other. She was offered a job with a different company and they have been emailing each other back and forth for a year (i found out on his computer) I know I shouldn't have been going into his emails but after I heard they were close and still emailing I freaked out. Oh and also he never once mentioned me in any of the emails. Am I overreacting? Is he cheating? or at the very least do you think he has feelings for this girl?