Thank you Falko for your reply, that generate automatically other 2 question:
- why Courier instead Dovecot (only for my personal skills)?
- For install Apache2 in the mail server only for RoundCube, it need more dependencies? PHP and MySQL are installed by default for ISPconfig.
At I-Chat, your ideas are good, I think you must start with 2 ProxMox Server in cluster, and after with the dedicatet services on the host.
Bye thank you at all