1. When using a Google Workspace server as the SMTP server, the error could arise if the default configuration settings of the Gmail account block less secure applications.

2. Incorrect credentials for the configured SMTP Microsoft Exchange Server account.

3. If security defaults are enabled in your Microsoft 365, SMTP AUTH is already disabled.

1. Use the Microsoft 365 admin center to enable or disable SMTP AUTH on specific mailboxes:

   a) Open the Microsoft 365 admin center and go to Users > Active users.

   b) Select the user, and in the flyout that appears, click "Mail."

   c) In the "Email apps" section, click "Manage email apps."

   d) Verify that the Authenticated SMTP setting is enabled.

   e) Once you're finished, click "Save changes."

For additional information and support, you can refer to the official Microsoft documentation linked below:

Reference - https://learn.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/authenticated-client-smtp-submission