pop3 warn message, how to?

I´ve seen lot of providers for example when you send a message through outlook express and the domain is not allowed for relay, an error window appears giving the outlook error code 0x000xxx and some info like this domain is not in our relay list. And the message remains in the outbox folder.

Well i want to do this when people forge e-mail address with domains like hotmail using the smtp of the server, I think it should be done in antivirus exim, but I dont know how to.
I have do it in two ways, one delete the email and the other sends back a bounce telling him that we know what he is doing.

I dont know if there is a warn command or something like that.

If anybody knows, please post.

Thanks!