Exchange 2010: “You can't send a message on behalf of this user unless you have permission to do so”

An user had “Send-As” permission on a shared mailbox. But user couldn’t send e-mail as “shared mailbox”.  User is doing it correctly (add the shared mailbox name in From field). But user gets this bounced message:

You can't send a message on behalf of this user unless you have permission to do so. Please make sure you're sending on behalf of the correct sender, or request the necessary permission. If the problem continues, please contact your helpdesk.

If user tries OWA to send the e-mail as the shared mailbox, it may work. If it is,  then it’s Outlook issue.

Solution: Have user update the offline address book (click Send/Receive tab, click Send/Receive groups and select Download Address Book). Better yet,

1. Close Outlook
2. Delete the offline address book folder under “C:\Users\username\AppData\Local\Microsoft\Outlook\Offline Address Books”  (I assume it’s Windows 7 computer, look under C:\documents and settings\username\…. for XP computers).
3. Open Outlook and let it download new OAB.

Other Possibilities are,

• You just gave “Send-As” permission for the user. Then, you have to wait for few hours. (you may restart Information Store to take effect the permission right away, who wants to do it? )
• User’s Outlook got bad/outdated cached contact information. Search for *.NK* files under user’s profile and delete it. Obviously Close the Outlook first before you delete the *.NK* files.

If it helped you, drop a reply to this blog.