Exchange 2010/Outlook 2010 MailTips issue

Hi,

A user is trying to send an email to another valid internal user, MailTip shown the following message:

“This e-mail message cannot be delivered to Doe, John X because the e-mail address is no longer valid”

(This symptom is happening to me as well).

I have tried the following:

• Open Outlook 2010 with Cached or Non-Cached mode.
• “Empty Auto-Complete List” by doing it in File, Options, Mail, Send messages.
• Select this account from the GAL.
• Deleted all *.oab files in Offline Address Books folder

With this warning message, the email is still delivered successfully (received the delivery receipt)

This problem does not happening in Outlook WebApp (OWA). Yes, MailTips feature works in OWA that has been verified.

Does anyone have soltuion? How can we verify the MailTips feature working properly not given false information?

Thanks,