My husband is secretly sending sexual emails to a coworker, what do I do?

My husband does not know that I know that he sends these emails to a coworker. He says stuff like, "you're hot", "you're sexy", "are you doing to wear a little black dress for me?" It is so devastating for me to see this! If he found out I snoop through is emails he would be so angry. I don't think he's ever cheated on me but these emails are inappropriate. How do I tell him I know without telling him that I look through his emails? Please help me.