Microsoft Knowledge Base Article 264035 contains the following summary:

You can configure an Exchange Server computer to use specific Transmission Control Protocol (TCP) ports for the information store, directory, and System Attendant to enable access to it through a firewall, router, or other device that blocks certain TCP or User Datagram Protocol (UDP) ports. You cannot configure the ports that will be used when the server sends a client a new mail notification packet. In a situation where UDP traffic from the server to the client is blocked, clients may not receive new mail notification.

For additional information about configuring TCP/IP access to an Exchange Server computer through a firewall for Exchange Server version 5.5, click the following article number to view the article in the Microsoft Knowledge Base:

148732 Setting TCP/IP port numbers for Internet firewalls.

For additional information about configuring TCP/IP access to an Exchange Server computer through a firewall for Exchange 2000 Server, click the following article number to view the article in the Microsoft Knowledge Base:

270836 Exchange 2000 static port mappings.