Which protocol is primarily used for sending outgoing mail?

The protocol primarily used for sending outgoing mail is Simple Mail Transfer Protocol (SMTP). SMTP is designed specifically for the transfer of emails from a client to a server or between servers. It operates by instructing the email server on how to send the email to its destination, utilizing a push mechanism to deliver the message effectively.

In contrast, IMAP (Internet Message Access Protocol) and POP (Post Office Protocol) are primarily used for retrieving and managing emails from a server. IMAP allows for multiple devices to access and synchronize the same mailbox, enabling users to manage their emails in real-time. POP, on the other hand, downloads emails from the server to a single device, typically deleting them from the server after retrieval.

HTTP (Hypertext Transfer Protocol) is primarily used for transferring web pages rather than handling emails in any way. Therefore, SMTP stands out as the correct answer for outgoing mail transmission.