Is there any way to use policies to enforce a limit on mail size? I know that this limit can be enforced at the router on the server, but by this time, the email has transferred to the server.  

Ideally, I would like a policy whereby a user was warned if they attempted to send a message over x MB (are you sure you want to send this message, it may take a long time to send) and that would not let them send at all if it were over x MB. (the message you are sending is over ___ MB, and can not be sent.  Please remove some attachments and try again)

Is this possible? 

Thanks

Sam