a friend of me did use my xmpp address @jit.si as an Email address because
there is an @ sing in it.
Ok, my bad to not explain it from ground up. But! The server behind @jit.si
happily takes emails, and do not forward/ translate them to xmpp messages
or report an error. So I was unaware of the mistake, and my friend was
unaware of the mistake.
Possible solutions: Report error to user who sends emails, or close the
ports, or translate incomming emails to xmpp.