M365 - Exchange - This message could not be sent. You do not have the permission to send the message on behalf of the specified user.

When you using "Send As", you receive the following error message:

This message could not be sent. You do not have the permission to send the message on behalf of the specified user.

Solution:

In Office 365 and go to Exchange Admin Center(EAC) search,  click and edit the shared mailbox. Then go to mailbox delegation, add your account to the Sent As permission list. Then wait a few hours to take effect. Then you will be able to send emails as the shared mailbox. In Outlook 2016 client, click From and choose Other Email Address and type the address of the shared mailbox.

 

Reference:

https://support.microsoft.com/en-us/office/open-and-use-a-shared-mailbox-in-outlook-d94a8e9e-21f1-4240-808b-de9c9c088afd

https://answers.microsoft.com/en-us/outlook_com/forum/all/send-mail-as-a-shared-mailbox/074b9d20-16c9-4431-bb88-6ce81331844f

https://copyprogramming.com/howto/send-as-vs-full-access-shared-mailbox

M365 - Exchange - This message could not be sent. You do not have the permission to send the message on behalf of the specified user.