I've granted one user SendAs and FullAccess to two shared mailboxes. One mailbox works fine, the other generates an error when attempting to SendAs:
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.
/O=ORG1/OU=EXCHANGE ADMINISTRATIVE GROUP (FYDIBOHF23SPDLT)/CN=RECIPIENTS/CN=<USER1>
I've omitted company specific information from the error and replaced with, USER1, DOMAIN, and SERVER.
Here's what I've tried so far:
1. Deleting/readding SendAs permission
2. Deleting all AutoComplete/NK2 entries in Outlook
3. Verified neither of the mailboxes are hidden from GAL
4. Updated the OfflineAddressBook and FileDistributionService
5. Closed Outlook and deleted all OAB files in user's Outlook folder
6. Restarted Outlook and d/l a fresh, full OAB
7. Verified the proper Identities were included in the msExchDelegateListLink attribute using ADSI Edit
After all this, the user still gets the NDR after trying to SendAs. When attempting SendAs in online mode, the email routes properly and no NDR is generated so this issue appears to be specific to the Outlook client.
Any suggestions would be much appreciated.