[Issue Resolved]: You are not authorized to send mail on behalf of the specified sending account.

Introduction

Recently, while working on a Power Automate Flow, we came across a requirement to send an email from another user. We used the “Send an email (V2)” action.  In this action by clicking on “Show advanced options” we configured the From (Send as) field.

But the flow run failed. The error message below was displayed:

"You are not authorized to send mail on behalf of the specified sending account."

This means to use this option we are required to have "Send as" or "Send on behalf" permissions granted to the account using which the “Send an email V2” action is configured. 

In this blog, we will learn how to set up required mailbox permissions in order to use From (Send as) option.

Solution:

Step 1: Go to the Admin Center.

Step 2: Select Users > Active Users 

Step 3: Select the user for which you want to manage the mailbox permissions.
Step 4: A popup will appear and on the Mail tab you can see 3 different permissions under Mailbox permissions. 

• Read and manage permissions: This permission allows the assigned user mailbox to read as well as manage emails in the user mailbox on which the permission is assigned. 
• Send as permissions: This permission allows a user to send emails from the mailbox as the mailbox owner.
  
• Send on behalf permissions: This permission allows the assigned user to send emails on behalf of the owner.
  
  
  
  

Step 5: To configure the permission, click on the respective permission and select Add permissions. In this case we wanted to grant “Send as” permission. 

Step 6: Now select the user (Flow Creator) you want to grant permission to. 

Step 7: Select Add.

Note: Once you've set up the permissions, it can take up to 60 minutes for the changes to be in effect. 

Conclusion:

So this is how you can send emails on behalf of another user and send emails as if they were sent from that mailbox.

 If you have any questions you can reach out our SharePoint Consulting team here.