Ok. So you don’t want him to suspect that it came from you. Personally, I agree with Sara that he needs some humiliation, but I get where you’re coming from.
So how about this. Wait a few weeks. Don’t say anything else to your husband about this topic for now. Then, set up an anonymous email account and send your imam a message. Make sure that you leave out anything that identifies you, including the fact that you are female. Just say that you are a concerned member of the congregation who has noticed along with many others that both men AND women (yes…women do this too) are being indiscreet about their intimate lives. Request that the imam do a khutbah or presentation about the topic. And then ask him to get back to you regarding a date, assuming he agrees to speak about this issue.
That way, the imam will have no way of knowing who has sent this - male or female. He won’t be able to connect it with anyone. And hopefully, he’ll mention that it’s becoming a widespread problem (which it is) and that should, Inshallah, allay any suspicions your husband may have about who sent it. Even if he does suspect and you feel that you have to come clean, you can tell him that the Imam doesn’t even know whether it was a man or woman who sent the message, much less the exact person the message concerns.
Hope that helps.