Is this considered cheating?

I came across a 6 month old email conversation between my boyfriend and another woman. I wasn'r snooping at first but the name of the woman caught my eye (she lives far away but was considering relocating here) and I read the email. She asked him if he were single and he replied. "Single enough" although this is not at all true. We have been together for 5 yrs and he recently asked me to move in again as we had stopped living together last year but continued our relationship. There has been no further correspondence since this email 6months ago, as far as I know. Months ago I found this woman in my boyfriend's IM contacts. When I asked him who she was, he told me he had no idea. We have had a volatile relationship but both of us have agreed that we are the love of each other's lives and really like spending most all our time together. I can see that he is genuine in his face and his actions but for this email in which he so clearly lied to both her and me. What do you think? Is or has he been unfaithful? What would you do? He will deny it of course.....