Men please answer--found out my husband emails a female coworker at work?

I recently found out my husband emails a coworker (Karen) at work. I went to a party with my husband and someone there (Lily) told me that my husband is emailing a coworker at work. She is friends with Karen and said she is really nice. I met this coworker he has been emailing & we talked briefly. She seems like a nice girl and she is sort of cute. My husband introduced us, sort of, but didn't tell me her name or tell him mine. He didn't look sneaky at all when he did this. He then put his arm around me. He was attentive to me at the party and did talk to her a few times separately from me, but just for a couple of minutes. Nothing seemed fishy to me. I asked and Lily forwarded me Karen's emails to my husband. Sometimes they're just group emails, but he sure is attentive to her and there is some innocent tease/flirt going on. Lily said that she felt compelled to tell me this, as this woman really has a crush on my husband. MORE--see added details I will be now adding.