[Send As permission issue with on premise mailbox due to auto mapping] Removing an Auto-Mapped Mailbox from the Outlook in Office 365 hybrid environment.

EXO Migrated user not able to send email from on-premise hosted Shared Mailbox 

Problem:  User whose mailbox has been migrated to Exchange Online is not able to send email as shared mailbox currently hosted in On-premise Exchange.

Root Cause: These mailboxes were mapped using the full mailbox permissions with the Automapping feature, to fix the issue auto mapping needs be removed and the shared mailbox should get disappear from the outlook.

How to fix:
To fix this issue remove the shared mailbox permissions and add back using the -Automapping: $false switch.
Add-MailboxPermission -Identity SharedMailbox -User USERID -AccessRights fullaccess -AutoMapping:$false

Besides removing and reapplying permissions we also need to remove the user entry from the msExchDelegateListLink attribute.

Following the below steps.
Open the Active Directory Admin Center and search the shared mailbox, browse the properties, click on Attribute Editor, look for “msExchDelegateListLink”.
Now remove the problematic user entry from the list.
This may take a couple of hours before the shared mailbox gets disappear from the outlook.

Once the shared mailbox disappears from the outlook, add the shared mailbox as an additional mailbox, This should fix the Send as permissions issues.