What protocol is primarily used to send email messages?

The protocol primarily used to send email messages is SMTP, which stands for Simple Mail Transfer Protocol. It is specifically designed to facilitate the transfer of email between servers and from clients to servers. When an email is sent, SMTP handles the communication by initiating the session between the sender's and recipient's email servers, ensuring that the message is properly routed and delivered to the intended inbox.

SMTP operates over the Transport Layer and generally uses port 25, although it can also function over ports 587 and 465 for secure transmission. This protocol is vital for email delivery, making it a fundamental part of email communications on the internet.

In contrast, the other options perform different roles in the realm of network communications. ICMP (Internet Control Message Protocol) is primarily used for sending error messages and operational information, such as when a service is unavailable or a router cannot be reached. POP3 (Post Office Protocol 3) is used for retrieving emails from a server, allowing users to download their messages but not to send them. FTP (File Transfer Protocol) is utilized primarily for transferring files rather than sending emails. Each of these protocols serves important functions, but for sending email specifically, SMTP is the designated protocol.