Well a customer asked if I could set this up for him. I've never been asked this before and there is probably a good reason for it..

However: this client wants to only receive email that comes from about 20 email addresses and wants all the other emails he gets to either bounce or just have the server delete this.

Is this possible?

You could do that with an email alias piped to a script in sendmail, or with dot-qmail in qmail. And of course a script to parse the messages for the required sender addresses and take the required action.

Seems that it would be easier for qmail to actually bounce the messages, although sendmail or qmail could drop the messages to /dev/null equally well.

Or install procmail

www.procmail.org