Email undeliverable to Exchange 2003. Requested action not taken: mailbox unavailable

I would start by getting rid of the apostrophe. Many other mail systems can have issues with these so I make it best practice not to use them in SMTP addresses otherwise from time to time you will get NDR's from certain external companies. You can test if this is the issue by adding another SMTP address for this user without the apostrophe and then have the person with the issue re-send to that address.

The apostrophe would be a flag for me.
What I tend to suggest is that you create a second email address without the apostrophe and make it the default. Then if someone does use the apostrophe it continues to work correctly.

Simon.

Ok. Let me clarify.  The email address with an apostrophe is someone trying to send email to my exchange server. So I cannot change their email address.  However, the emails are rejected by our server from that individual.  How can I resolve this?

There is very little that you can do.
Apostrophes are a problem and any decent email administrator will not use them.
Put that email address in to a file for importing etc and it will cause havoc.

I doubt if it is Exchange blocking the message, it is probably AV or antispam software.

Simon.

The person that has the apostrophe in his email address, is able to send to anyone else, just not to our email server.  In addition to or Exchange 2003 server.  We have a Watchguard firewall with spamblocker service and we have Sunbelt Software's Messaging Ninja software installed on our exchange server.

Any ideas on what we can do to allow his emails to be delivered? It is obviously something on our end.

Watchguard!