How to make secure SMTP Relaying

[ml at bortal.de]

Hello List,

we currently have 3 E-Mail Boxes running sendmail and cyrus. We serve
about 50 customers and 800  e-mail adresses. The Customers usually have
an Exchange-Server or directly connect to us using smtp auth.

Out problem: If a customer sends out an E-mail bomb by accident or
purpose, then the ip-address of our mailserver will be blacklisted!

Whats the best way to avoid this? Is there a way to set some sort of
limit or something?

Example: If a customer with 50 E-Mail Adresses gets a LOT of spam
including unknown user bounces, then i will get a lot of traffic wich
will look like some e-mail attach, too.

Any ideas?

Thanks,
Mario