Thanks for you reply. We want to use POP3 (not SSL POP3) and only port 110 is open on the firewall.
The problem is that we can download messages ONLY if the "enable clear text login" is enabled, otherwise POP3 users receive the error "only valid after entering TLS mode".
Of course I don't like to be forced to use the "enable clear text login" option to make POP3 work.
I don't really understand what I did wrong

Thanks for your help
Simone