Hello Emanuele! This behavior points at an issue during SMTP communication with the end user's email server. Can I have you check the SMTP logs from Settings>Diagnostics>Log Files for messages relating to these email addresses and let me know what you see? My theory is that the receiving server may not be terminating the session as expected and this results in the bounce notification. Lastly, if you are not on our latest release I recommend giving this a shot as well. Hope that helps!

Hi Kyle,

we are using Build 7265, I'll upgrade to the latest and then I'll check again.

Thanks.

Hi  Kyle,

I did the upgrade at the latest version, but this problem persist.

Here a screen of SMTP log.