Hi,

Background:
I set up a new Small Business Server 2003 (Exchange Server 2003 SP2) for a client recently - this is the only server in their new Active Directory 2003 domain, domain1.local.  As well as setting up Exchange for domain1.com, they also have domain2.com, which I have added to the Default Policy under Recipient Policies - appropriate users have had a domain2.com email address added to their Active Directory user account and all seems fine internally.  I changed their MX records last Friday night so that incoming mail is now delivered directly to the in-house Exchange Server, 'server1'.  I sent some test emails from my MSN email address and they were received into the Exchange Server and put into the appropriate user's mailbox in lightning speed.

Problem:
HOWEVER, on Tuesday morning (Monday was a public holiday here), I learned that most senders cannot get an email through to either domain1.com or domain2.com - the non-delivery report that is returned states:

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:
  user@domain1.com
    unrouteable mail domain "domain1.com"

or

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:
  user@domain2.com
    unrouteable mail domain "domain2.com"

I've Googled this and everyone seems to be talking about a DNS issue, probably with the MX records being wrong.  So we've been messing about with the MX records over the past couple of days so that the MX points to the IP address or, alternatively, points to server1.domain1.com and then has a name record stating that server1.domain1.com refers to the valid IP address for it.

This does NOT seem to be the problem.  Incidentally, domain1.com's DNS records are still in the process of updating (and are currently wrong) so I'm only troubleshooting domain2.com at the moment.  I've done a DNS test (http://www.checkdns.net/quickcheck.aspx?domain=blacksigma.com&detailed=1) and it says that the MX records were correctly configured, the mail server responded on the correct IP address and it was perfectly happy to receive a test email message to info@domain2.com.

It certainly looks like an Exchange configuration issue.  Some senders CAN get an email through to these domains (I tested it on Friday night using my MSN webmail account) but the significant majority get the "unrouteable mail domain" error, including myself when I send it from my own Exchange Server at my office.

Any ideas what I could have done wrong?  It's both domain1.com and domain2.com that were having the same problem earlier in the week.  

Could it be anything to do with the fact that I tend to use sentence case (e.g. Domain1.Com) when I type?  I would be surprised (and really stuck!) but the reason that I ask is that I know that I've set up Exchange with entries typed in sentence case but I had trouble using telnet earlier (to be honest, it was the first time I ever really used it) because it was case sensitive.

The client is two hours away but I have remote control over the server from here.