What is the TCP port number associated with SMTP?

The correct answer is that the TCP port number associated with SMTP (Simple Mail Transfer Protocol) is 25. SMTP is the standard protocol used for sending emails across networks. It operates primarily on this port for communication between mail servers and clients. When a client sends an email to a mail server, it typically uses port 25 to establish the connection.

Understanding the importance of port numbers in networking is crucial, as different services operate over specific ports. For example, port 110 is associated with POP3 (Post Office Protocol), which is used for retrieving emails, while port 443 is designated for HTTPS (HyperText Transfer Protocol Secure), a protocol for secure web communications. Port 80, on the other hand, is used for standard HTTP traffic. Each of these ports is critical for their respective protocols, but SMTP specifically relies on port 25 for its operations.