Which protocol is primarily used for electronic mail transmission?

The primary protocol used for electronic mail transmission is Simple Mail Transfer Protocol (SMTP). SMTP is specifically designed for sending and relaying email messages between servers. When an email is sent, it typically goes from the sender's email client to the sender's mail server using SMTP. The mail server then relays the email to the recipient's mail server, allowing for efficient message delivery across the internet.

In contrast, while other protocols are crucial in the email system, they serve different functions. For instance, IMAP (Internet Message Access Protocol) is used for retrieving and managing emails from a mail server but does not handle the actual sending of the emails. SNMP (Simple Network Management Protocol) is a standard for network management and monitoring device health, and TCP (Transmission Control Protocol) is a foundational network protocol that facilitates communication but is not specifically tied to email communication. Thus, SMTP is the correct answer, emphasizing its central role in the transmission aspect of electronic mail.