Bastille
21, 22, 23, 25, 53, 80, 110, 443, 8443, 993, 995
21 - FTP
22 - SSH
23 - Telnet (you should really close this if you have SSH installed)
25 - SMTP
53 - DNS
80 - HTTP
110 - POP3
443 - SSL
8443 - PSA
993 - Secure IMAP
995 - Secure POP3
You might want to open 143 which is IMAP4 (non SSL version)
21, 22, 25, 53, 80, 110, 443 and 8443 are all pretty much required to use your server.