Yeah, I think that's something other than the "Allow Users to Email Other Members" feature. Do you have any way to look at your server's access logs? If you have a really busy forum it might be hard to find the requests that sent the email, but maybe you'd be able to notice lots of access to the same script in a short time.

I checked the access log and didn't see anything out of the ordinary, mostly showthread and standard pages like this.