Change email domain name

I have a company that just got a new Exchange Server, it's running on a SBS 2003. Before the
exchange server arrived, they were using an e-mail host which the client pc's were accessing via POP3. They're still using the POP3, but now delivering to the exchange mailboxes (instead of PSTs). There is about a dozen users.

One of the users is having a problem, when she sends out meeting requests, the 'from' says:
From:Joe Schmo [jschmo@productxchange.com]; on behalf of; Joe Schmo [jschmo@productexchange.com]

The 'on behalf of' spelling of her email address is wrong and if you hit reply, it uses the  incorrect one. The correct spelling of their email domain is productxchange.com, (no "e" after product) not productexchange.com.

So I guess the simple question is how do you change the users email address.

Further Detail
I looked in "Active Directory Users and Computers" at the Joe's properties, on the general tab
under the email entry, the domain is spelled wrong and under email addresses tab it has both
addresses listed. I've tried editing the domain name, taking out the 'e', and I've tried setting the correctly spelled version as default under the email addresses tab, but if I hit 'Ok' and then go back into the properties, it's again spelled wrong.
 
I looked at other users, and they too had incorrectly spelled email addresses, but they haven't
reported problems. I don't know if this is because they're just not sending out calendar events
or what. The problem only seems to show up when you invite people to calendar events, not when
you send email.