I just received an anonymous e-mail from one of my "friends" saying my husband is cheating and has been for a while. She saw him at an apartment building, and the front-desk person indicated that he is the boyfriend of someone who lives there. I am summarizing, but the e-mail writer indicated that she used an anonymous e-mail account because she doesn't want to jeopardize our friendship.
There was enough detail to suggest this is someone who knows me and my family. So now I am struggling with what to do with this information.
Answer: Oh wow, I really feel for you - the anonymous tip is a hideous way to get bad news.
If this particular coward is checking her (I use your pronoun) fig-leaf e-mail account, though, at least you can ask follow-up questions. Anonymous notes preclude even that.
When you do ask the follow-up questions, also point out that the decision to remain anonymous has jeopardized all your friendships now, because you will have to regard all of your friends with suspicion of being the anonymous source. Then say you would be grateful to her for showing the courage to come forward.
And, finally, you need to show the e-mail to your husband - at some point. If you have reason to believe he'd respond poorly or play dirty (be brutally honest with yourself), then talk to a lawyer first to make sure your T's are crossed and your bank accounts and other valuables are safe.