When one of my client try to reply or send new email to me, he got an NDR like the one below: The e-mail address could not be found. Perhaps the recipient moved to a different e-mail organization, or there was a mistake in the address. Check the address and try again. <fa-sbs.FA.local #5.1.7 smtp;550 5.1.7 The domain you are attempting to send mail to is invalid.> I'm running exchange 2003 server. What can I do to fix this. It's only happen to this client but everyone else seem to be fine. Is it on my end or his. Please help. Thanks in advance

I would ensure that your e-mail server has a reverse DNS lookup address. I would then have the client try to do an nslookup on your server. It seems like the other person's server is not finding your domain. If they are trying to validate your domain addresses using reverse DNS lookup, and it is not configured, it may fail. Also, if someone at the other location tries to connect from the command prompt using telnet on port 25 to the IP address of your server, does it work? If so, does it fail if they try to use the A record that your MX record points to?