Damn spammers pretending to send from a clients email address

Hi,
This is more a network admin type question but duno if anyone has any ideas.

Duno if this has happened to anyone else before but a spammer has got hold of one of my clients email addresses and is using it as the "from" address for a load of spam. The client then gets a load of non delivery reports. This happened to me a while ago when I had a hotmail address and I got something like 2000 non delivery reports one day.

The messages definatly are not being sent from my email server and I have checked its not an*open relay etc.

I am worried that the client (or my email server) will get blacklisted because of this if the spam checker programs dont check where the email actually originates from.

Apart from creating an inbox rule is there anything you can do to stop this kind of thing?

Thanks,

Alex