Which protocol is primarily used for server-to-server email transfer?

The protocol primarily used for server-to-server email transfer is Simple Mail Transfer Protocol (SMTP). SMTP is specifically designed to manage the sending and routing of emails across the internet. When an email is sent from one server to another, SMTP handles the process of transferring the message, ensuring it reaches the intended recipient's email server.

While other protocols like IMAP and POP3 are involved in email retrieval—IMAP allows users to access their email on various devices synchronously, and POP3 primarily downloads emails to a single device—neither is concerned with the transfer of emails between servers. FTP, or File Transfer Protocol, is unrelated to email and is used for transferring files between computers on a network. Therefore, SMTP is the clear answer for server-to-server email transfer.