Code:
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 25 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 80 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 443 -j ACCEPT
These rules will allow SMTP, HTTP and HTTPS into your server. All outbound should be enabled by default anyway.
Note: You will probably have to change the IPTables chain name for whatever your inbound one is.