Solution: Outlook error “because the email address is no longer valid” but actually mail can be delivered to user.
Applies to Exchange 2010 and Outlook 2010
When you try to send mail user Deniz Ata (email@example.com) , you placed the user in TO field you got the following error on Outlook mail tips section but actually the mail can be sent to user. Deleting the recipient from Outlook name cache does not work.
“We won’t be able to deliver this message to Deniz Ata because the email address is no longer valid”
mail attribute of the user is different from the primary email address (reply address) in email tab in Exchange 2010 console. That is ProxyAddresses attribute of the user object:
1-Open Exchange 2010 console, find the user
2- E-Mail Addresses tab, uncheck “Automatically update e-mail addresses based on e-mail address policy” if necessary
3-Select primary SMTP address, click edit and set to firstname.lastname@example.org. Click OK
4-Wait for few minutes and try again. If there is still problem, on client site close Lync Client and Outlook and open Outlook and try again.