Hybrid Exchange: Cannot SendAs or SendonBehalf of mailbox when user mailbox has been migrated to Exchange Online but sending mailbox remains on-premises

In Hybrid Exchange scenarios, if an Active Directory user mailbox has been migrated to Exchange Online, but a user mailbox or shared mailbox that the user can SendAs/SendonBehalf has not been migrated, the user may experience and error when trying to send a message as the on-premises mailbox.

Such a message will bounce back with an error message like the one in the below image.

This occurs because SendAs/SendonBehalf permission does not synchronise to the MailUser object in Exchange Online for a mailbox that has not yet migrated.

To fix the issue for SendAs rights:

1. Connect to Exchange Online PowerShell.
   
   
2. Add Send As trustee permission to the MailUser object. You can use the command similar to:
   

   Add-RecipientPermission -Identity non-migrated-mailbox@organisation.com 
   -AccessRights SendAs -Trustee person-to-sendas@organisation.com

Repeat this for each user that has Send As permission granted for on the on-prem mailbox.

Once done, Exchange Online will be able to allow a message to Send As correctly for the MailUser object for the message that originates in the cloud - and Exchange Server will use its Send As settings for messages that originate on-premises.

To fix the issue for SendonBehalf rights:

1. Connect to Exchange Online PowerShell.
   
   
2. Add SendOnBehalf permission to the MailUser object. You can use the command similar to:
   

   Set-MailUser -Identity non-migrated-mailbox@organisation.com 
   -GrantSendOnBehalfTo person-to-sendas@organisation.com