Outlook SENDMAIL shows wrong email address

We have numerous CONTACTS in our Exchange Global Address List (Exchange Server 2007), who work for a company whose domain name changed.  When we first setup these contacts, their email addresses were something like   username@firstdomainname.com   When they changed their domain name, we updated all the associated CONTACTS so that their email addresses reflect the new domain name, and so they now read something like   username@seconddomainname.com

When our staff creates a new mail message, it IS delivered to the correct address.  Also, if they enter the contact's name on the "To" line of a new mail message, and then right-click on it and select "Outlook Properties" it DOES show the new, updated email address.  HOWEVER - if on that same mail message the user right-clicks on the contact's name and selects "SENDMAIL" then it still shows the OLD email address.

I have checked the contacts on the server, and I see no reference whatsoever to the old email addresses.  However, it must exist somewhere, or else it wouldn't be showing up undering "SENDMAIL" in Outlook.  Our staff apparently uses this as a "quick method" to look up (and thus pass along) a contact's email address, and so they have inadvertently been giving out incorrect email addresses.

What do I need to change on the Exchange Server so that it gets rid of all references to those old email addresses?

Thank you,