Which one of the following ports is not normally used by email systems?

The port that is not normally used by email systems is 139. This port is associated with the Server Message Block (SMB) protocol, which is primarily used for file sharing and network printing in a Windows environment. Email protocols, on the other hand, commonly use different ports for their specific functions.

For instance, port 25 is typically used for outgoing mail transfer via Simple Mail Transfer Protocol (SMTP), while ports 110 and 143 are used for incoming mail retrieval. Port 110 is used for Post Office Protocol version 3 (POP3), which allows email clients to retrieve emails from a server, and port 143 is used for Internet Message Access Protocol (IMAP), which enables email clients to access and manage their emails directly on the mail server.

The distinction is important when considering network configurations and firewall settings, as certain ports need to be open to allow proper email functionality while port 139 is irrelevant in this context. Understanding these specific functions associated with each port helps in grasping how email systems operate within a network effectively.