Have you ever had email from one specific domain bounce back from Exchange with status code of: #550 4.4.7 QUEUE.Expired; message expired ##? I ran into this problem. It turned out the mail server on that domain was blocking email coming from our Exchange 2007 mail server. Here is what I did to narrow down the issue.
I went to http://www.mailboxtools.com and used a MX server look up tool to find out the mail server of the remote domain. You can accomplish this same thing using nslookup. Once I found the mail server on the remote domain tried to telnet to their mail server on port 25 from our Exchange mail server. Example:
c:\ telnet mail.server.com 25
It gave me this result:
554 Too many connections from origin
Connection to host lost.
I tried the same telnet command from a Exchange 2007 mail server I have on a test network (different external network). This telnet worked. I knew then our mail server was being blocked. I contacted the remote domins admin explained the scnerio and sure enough their Barracuda SPAM appliance was blocking our domain for some reason. They whitelisted our domain and has worked flawlessly since.