The Internet Mail Connector and the Internet Mail Service use TCP port 25 for inbound SMTP messages as defined by RFC-821. For inbound SMTP messages, the Internet Mail Connector and Internet Mail Service monitor port 25 for incoming connections from other SMTP hosts. Communication between IMAP4 clients and Exchange Server computers Exchange version 5.5 supports IMAP4, the Internet Message Access Protocol. IMAP4 is a superset of POP3 and therefore supports all its features and some additional ones. An example of an IMAP4 enhancement over POP3 is the ability to search messages for key words while the messages are still on the mail server.
Users can then choose which messages to download to their local computer.
IMAP4 also allows access to public folders and personal folders. The ports that IMAP4 clients use when accessing messages on an Exchange Server computer depend on the authentication method in use. With Basic or NTLM authentication and TCP, the IMAP4 server listens on TCP port 143 for any incoming connection requests from IMAP4 clients for message download and retrieval. If SSL authentication is used, however, the port on which the Exchange Server computer listens is TCP port 993. Router and firewall setups should therefore take into consideration the access to TCP port 143 or TCP port 993 when this protocol is a supported feature for messaging.