I am new to this forum, but I wanted to share with you a solution to a problem that I recently resolved. This problem had prompted me to sign-up for msexchange.org, however, I found an answer to my problem before I was able to create my "problem post". If my post is redundant I apologize, but due to the amount of time and effort I put into getting a solution I figured someone might benefit from this post. I am not an Exchange administrator by any stretch of the imagination, I support the Outlook application.
We had a user go from Office 2003 to Office 2010. Whenever this person would address e-mails to certain people they would get that pesky mail tip that would say "This e-mail message cannot be delivered to (recipient name) because the e-mail address is no longer valid". This message would appear even when they would be selecting a recipient out of the GAL. Another Outlook 2010 user could address a message to the same user out of the GAL and not receive the message.
I ripped my hair out over this problem. I deleted/recreated the user's mail profile, could not duplicate the issue in OWA, ran a repair on the Office installation, etc.
To resolve this issue we had to delete the user's nickname cache. In earlier versions of Outlook the nickname cache would reside within the user's profile as an .nk2 file on the machine they were using Outlook on (%appdata%\Microsoft\Roaming\Outlook\) Now, in Office 2010 the nickname cache is stored as part of the mailbox on the Exchange server. To delete the cache have the user do the following in Outlook: Click File, Click Options, Click Mail, scroll down to the Send messsages section. Click the button that says Empty Auto-Complete List. Click Yes on the message that asks if you are sure you want to empty the auto-complete list. Click OK on the Outlook Options window, exit Outlook, then re-launch the application.
The user should now be able to select from the GAL and not receive the invalid e-mail address message, unless in fact, the e-mail address is truly invalid. Yes, it's true their drop-down list when addressing messages is gone and will need "rebuilt" but they shouldn't be using that as an "address book" anyway - if you need the e-mail address in the future add it to your contacts in Outlook - easier said than done, right?
You can see Microsoft's article on this issue here: http://support.microsoft.com/kb/287623