Is it reasonable to ume my husband "Jim" is having an affair with a co-worker?

Suspected my husbands recent behavior changes (including a new sport car) and I opened (okay -hacked into) his email. He has numerous emails via blackberry to and from a co-worker "Kathy". He signs the emails " L U - Jim" and calls her babe or baby. And , oh yeah, & complains that his wife doesn't understand him. I confronted him and he swears they are just close friends and he would never cheat. Huh..yeaaah...I'll buy that......time for a lawyer, or a therapist, or a hitman...what do you think??