Yes sorry, I've already setup a DNS A record for mail.domain.com.
So at the end I've 2 DNS A records:
domain.com -> 123.456.789.012
mail.domain.com -> 123.456.789.012
You do not need any redirect as you added a vhost for this subdomain, if you added a redirect, then delete it.
That's the point. I want now use the ISPConfig to manage domains, so I've cancel from 000-default apache file the lines for that domain...
Anyway even like that it's not working.
If I restart Apache, I've this warning message:
Restarting web server: apache2[Tue May 25 15:27:47 2010] [warn] NameVirtualHost 123.456.789.012:443 has no VirtualHosts
... waiting .[Tue May 25 15:27:49 2010] [warn] NameVirtualHost 123.456.789.012:443 has no VirtualHosts