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Error sending message: Error sending message. MS365 returned status 403...

You'll need the "Send As" permission to send email from another email address in your organization. 

Error sending message: Error sending message. MS365 
returned status 403: The user account which was used to 
submit this request does not have the right to send mail on 
behalf of the specified sending account., Cannot submit 
message. 

First, let's confirm your Microsoft 365 connection

For Microsoft 365, we recommend that each user is connected to their individual ("Personal") email account like username@example.com.

  1. In Acctivate, click File > User Settings* and look at the Email Settings section
  2. Your email address should be listed as "Connected to: ..."  By default, Microsoft only allows you to send email from this email address.
  3. To send on behalf of a group or another user, your Microsoft 365 Admin will need to grant you "Send as" permission for that email address
  4. Your admin will need to use the following procedures depending on the mailbox type.

Instructions for a Distribution Group or Shared Mailbox

This is a common scenario for sending email from a distribution group like sales@example.com or support@example.com:

  1. Go to https://admin.exchange.microsoft.com and log in as an Admin user.

  2. In the left pane, go to Recipients > Groups

  3. You may need to click on the Group type (e.g., Microsoft 365, Distribution list, etc)

  4. Click the Group name to Edit the Group

  5. Under Members, click View all and manage members to add the user if not already in the list.

  6. Under Settings, Click Edit manage delegates

  7. Add a delegate for each group member who needs to "Send as" or "Send on behalf" of the group

  8. Click Save Changes

Instructions for a User Mailbox

This procedure allows one Microsoft 365 user to send email on behalf of a colleague's email address.

  1. Go to https://admin.exchange.microsoft.com and log in as an Admin user.
  2. In the left pane, go to Recipients > Mailboxes
  3. Click the Display name to Edit the mailbox
  4. Under Delegation, click Edit under the Send as section.
  5. Click Add members for each user who needs to "Send as" or "Send on behalf" of this mailbox
  6. Click Save Changes

🔄 Changes may take up to 60 minutes to take effect.

* File > User Settings was added in Version 14. You'll need an admin to open File > Configuration Manager > User > User Information for older versions.