Limit the number of outgoing e-mail

[softi]

Every now and then I experience that
olderly versions of cms are breaken into (like joomla, phpnuke)
and used to send ten of thousand of spam before

I realize the problem.
Therefore I would like to limit the number of
outgoing e-mail like many other companies already

do. Example:only 10.000 mails allowed monthly.

That should satisfy a normal user,
if somebody needs more, he can ask for that.

Anybody can suggest a good solution for that?

The server has debian etch installed on it.