We have open from the Internet trough a firewall to the Webserver the following ports:
TCP Ports:
21 ftp
25 smtp
53 DNS
80 http
110 pop3
443 https ( for user Sites)
8443 https (for Plesk Admin)
UDP Ports:
53 DNS
that should be enough from the internet to the webserver for working. of course the traffic back needs also to get trough. but if you chause a staitfull firewall this will be working by default.