[Exim] Spam (again)

Hello,

A little off topic and to stop Nigel, beating me with his virtual stick
perhaps replies (if any) should be private.

I have seen a trend (three times now), where a user sets up an SMTP mailer on
one of our dialup ports and then sends mail direct to an open relay somewhere.

A couple of our dialup ports are now coming up in ORBS as a result. Now I am
not too worried about that because in the end, customers shouldn't be sending
SMTP mail direct anyhow. I am worried about our reputation though and
wondered if, anybody had come up with any effective strategy against this?

--
Ian