How to refuse NDR and also grey list

[Happy]

I installed The Perfect Server - Ubuntu Lucid Lynx (Ubuntu 10.04) ISPConfig some months ago and it has been running fine up until lately. I am only using the following peices pop3/imap using Maildir format and therefore installed Courier-POP3/Courier-IMAP, Mysql, and Postfix. My mail server has been blacklisted several times lately and I am trying to correct the issues:
My mail server is not an open relay.

1. Users are getting a lot of NDR's -Is there a way not to accept kicked back mail on the mail server and just refuse it?
2. Also is there a way to do grey listing to reduce spam?


I tried getting spam assasin installed sometime ago with no luck.

Thanks