Sophos Central Email - Logging of missing mailboxes


I tried to find a way to see which User are missing a mailbox. 

We configured some domains which will be scanned by Sophos.

Now we get on some email addresses the error code ‎"550 5.7.1 Command rejected‎" in our exchange.

Which often means that there is mailbox in Sophos Central.

Is there a way to see in the logging or in a report which email are rejected because the mailbox does not exist.

Thanks for your help.