possible problem with MX records

I may have a problem with my email MX records or perhaps my Postfix setup. As I am totally out of my depth I need help determine what is up.

I host several websites on a VPS setup via Virtualmin and running Ubuntu 10.4LTS. One of the sites publishes a periodical newsletter. The previous newsletter sent in Feb 2013 worked fine but the most recent has about 10% of recipents bouncing back with a varity of 550 errors. Some of these address are known to be working ok. I need a guide on how to trouble shoot this issue as I dont even know where to start.

some of the return messages are:

host mx3.hotmail.com[] said: 550 Requested action not taken: mailbox unavailable (in reply to RCPT TO command)

host extmail.optusnet.com.au[] said: 550 5.1.1 name@optusnet.com.au: Recipient address rejected: User unknown in virtual mailbox table (in reply to RCPT TO command)

host mx.syd.iprimus.com.au[] said: 550 #5.1.0 Address rejected. (in reply to RCPT TO command)

host aln-mailrelay.att.net[] said: 521- blocked by sbc:blacklist.mailrelay.att.net. 521 DNSRBL: Blocked for abuse. See http://att.net/blocks (in reply to MAIL FROM command)

The first two of those error messages quite clearly state that the recipient address was unknown.

“Address rejected” can happen for various reasons; my guess though is that it’s also an unknown or malformed address.

The last message would indicate that your IP address is listed on a DNSBL blacklist, namely “blacklist.mailrelay.att.net”. Here’s a form to get you unlisted: http://rbl.att.net/cgi-bin/rbl/block_admin.cgi