Because Internet Connection Firewall examines all incoming communications, some programs, especially e-mail programs, may behave differently if you turn on Internet Connection Firewall.

Some e-mail programs periodically poll their e-mail server for new mail. Some e-mail programs wait for notification from the e-mail server.
Microsoft Outlook Express, for example, automatically checks for new e-mail messages when a timer tells it to do so. If new e-mail messages are present, Outlook Express prompts you with a new e-mail message notification. Internet Connection Firewall does not affect the behaviour of Outlook Express because the request for new e-mail message notification originates from inside the firewall.
Internet Connection Firewall makes an entry in a table that notes the outbound communication. When a new e-mail response is acknowledged by the mail server, Internet Connection Firewall finds an associated entry in the table and permits the communication to pass. You then receive notification that a new e-mail message has arrived.

Microsoft Outlook 2000 is connected to a Microsoft Exchange-based server that uses a remote procedure call (RPC) to send new e-mail message notifications to clients. Outlook 2000 does not automatically look for new e-mail messages when it is connected to an Exchange-based server.
The Exchange-based server notifies Outlook 2000 when new e-mail messages arrive. Because the RPC notification is initiated from an Exchange-based server that is outside the firewall (not by Outlook 2000), Internet Connection Firewall cannot find a corresponding entry in the table. Internet Connection Firewall does not permit the RPC messages to cross from the Internet to the home network.
The RPC notification message is dropped. You can send and receive e-mail messages, but you must manually look for new e-mail.