Does the user that is starting the SMTP service have access to the SMTP mailbox?
If the service is being started as the user that has access to the 'serviceintegration' mailbox, then it should work correctly. However, if the user starting the SM services does not have access to that mailbox, then the authorisation will fail.
So depending on whether this is a UNIX/Linux or Windows based installation, in the former the user starting the service must have access to the mailbox, in Windows the user set to start the service must have access to that mailbox.
If this is configured correctly then I would suggest that you check the mail server logs to see what they are reporting.