Which protocol is primarily used to send email over the internet?

The primary protocol used to send email over the internet is SMTP, which stands for Simple Mail Transfer Protocol. This protocol is designed to facilitate the transmission of email messages from a sender's email server to the recipient's email server. SMTP operates in a request-response manner and typically uses TCP (Transmission Control Protocol) on port 25, though it can also operate over other ports such as 587 and 465 for secure transmissions.

To elaborate on its functionality, when a user sends an email, the email client connects to the SMTP server and forwards the message through the protocol, which manages how the email is relayed to the appropriate destination. SMTP is specifically tailored for sending emails, making it the standard choice for this purpose.

Other protocols listed serve different roles in the email process. For instance, HTTP (Hypertext Transfer Protocol) is used for transferring web pages and other content on the web, while FTP (File Transfer Protocol) is utilized for transferring files across networks. Meanwhile, POP3 (Post Office Protocol version 3) is primarily focused on retrieving emails from a mail server rather than sending them. Therefore, SMTP is distinctly recognized as the protocol for sending email, underscoring its importance in internet communications.