Which of the following ports is used by SMTP?

The Simple Mail Transfer Protocol (SMTP) primarily uses port 25 for sending emails. This port is essential for email transmission between servers and for the relay of emails from a client to the server. SMTP is a communication protocol for electronic mail transmission, making port 25 the designated standard for this function.

Other ports mentioned serve different protocols: port 21 is associated with the File Transfer Protocol (FTP), which is used for transferring files; port 23 is designated for the Telnet protocol, primarily used for text-based communication over the internet; and port 80 is recognized for Hypertext Transfer Protocol (HTTP), which is used for web page delivery. Thus, understanding the specific role of SMTP and its designated port is crucial for anyone working with email systems and server configurations.