Under tweak settings I can set the maximum number of emails a domain can send in an hour.

The big problem with that is that if that domain tries to send 10000 emails and has a limit of 100 per hour, the exim mail queue has 9900 emails in the queue and jams the server!

How can I make it discard those emails?

Try this tweaks.

http://www.configserver.com/cp/cmm.html

:)

that's a nice tweak but it doesn't help me.