Which protocol is primarily used for email transmission?

The correct answer is SMTP, which stands for Simple Mail Transfer Protocol. This protocol is specifically designed for sending emails across networks. It is a text-based protocol that provides the necessary commands and mechanisms to send and route messages from one server to another, ensuring that the email reaches its intended destination.

SMTP operates on the application layer of the Internet Protocol Suite and uses port 25 by default for communication between mail servers. When a user sends an email, it often first goes through an SMTP server, which then takes care of the transmission and delivery of the email message to the recipient's mail server.

In the context of the other options listed, while IMAP (Internet Message Access Protocol) is related to email, it is used primarily for retrieving and managing messages from a mail server, rather than sending them. DNS (Domain Name System) is critical for resolving domain names to IP addresses, which facilitates communication over the internet but does not handle email transmission directly. HTTP (Hypertext Transfer Protocol) is used primarily for transferring web pages and other resources from a server to a client but is not used for email services.

This clarity about the different protocols shows why SMTP is clearly identified as the primary protocol for email transmission.