Emanuele,
In the screenshot you initially provided, every visible bounce comment appears to have been created within the same minute. To diagnose this, we would need to be able to see the raw content associated with several of these bounce messages, as well as review the general history of the ticket. It would also be helpful to be able to review the logs from the affected time period. We do not ask for this information through community threads, in case there is sensitive data involved. This is why we suggested, and still believe, a support ticket is the best way for us to be able to resolve your issue.
Andrew Barker
Senior Software Developer
SmarterTools Inc.
www.smartertools.com