This is just going to be a list of all the TCP & UDP IP ports that you could possibly need to use (or open in your firewall(s)).
TCP port 25 - SMTP
TCP port 26 - SSL secured SMTP
TCP port 110 - Post Office Protocol v. 3 (POP3)
TCP port 995 - SSL secured POP3
TCP port 143 - Internet Message Access Protocol v. 4 (IMAP4)
TCP port 993 - SSL secured IMAP4
TCP port 80 - HTTP for Outlook Web Access
TCP port 443 - SSL secured HTTP for Outlook Web Access (HTTPS)
TCP/UDP port 389 - Lightweight Directory Access Protocol (LDAP)
TCP port 3268 - LDAP communications with an Active Directory Global Catalog Server
TCP port 119 - Network News Transfer Protocol (NNTP)
TCP port 563 - SSL secured NNTP
TCP port 135 - Remote Procedure Protocol (RPC) however it will also use ports 1024 and up as needed.
TCP/UDP port 53 - Domain Name System (DNS)
TCP port 102 - Mail Transfer Agent (MTA) *Only used in X.400 connections.
TCP/UDP port 88 - KERBEROS
Additionally if you are secureing any servers such as front-end / back-end servers with IPSEC you will need to allow IP protocol 51 for the Authenitcation Header, IP protocol 50, TCP/UDP port 88 for KERBEROS, and UDP port 500 for Key Exchanges.
That's a pretty decent sized list "JUST" for one network service. On the plus side most of those are not needed in a default setup as POP3 and IMAP4 are rarely used in a pure Microsoft network.
Also hopefully this will help me remember some of the less used ports and let me ace my test!
-EDIT: Thanks to the commenter I updated this post to have the correct Global Catalog server port.