There was a few threads about problem with Office 365 mailbox used as support email. In “Both” in/out mode with Office365 mailbox you can see error about SMTP after successfully authorization. All threads in forum are closed without resolution.
This is not a Freshdesk error, but Office365 configuration issue. You need one fix in Office 365 Admin panel:
Use the Microsoft 365 admin center to enable or disable SMTP AUTH on specific mailboxes
Open the Microsoft 365 admin center and go to Users > Active users.
Select the user, and in the flyout that appears, click Mail.
In the Email apps section, click Manage email apps.
Verify the Authenticated SMTP setting: unchecked = disabled, checked = enabled.
When you're finished, click Save changes.
