What protocol is used between Email servers?

Sending an email from one person to another actually involves several different network protocols working together, each responsible for a different stage of the journey: getting the message from the sender to their mail server, moving it between mail servers, and finally delivering it to the recipient's inbox.

The Simple Mail Transfer Protocol (SMTP) is specifically the protocol used to send and relay email messages, including transferring messages between different email servers as the message moves from the sender's server toward the recipient's server. This server-to-server relay function is exactly what the question is asking about.

POP (Post Office Protocol) is used afterward, by an email client to download messages from the recipient's mail server to a personal device, which is a server-to-client operation, not a server-to-server one. FTP (File Transfer Protocol) is used for transferring files generally, not specifically for email. SNMP (Simple Network Management Protocol) is used for monitoring and managing devices on a network, and has no role in transferring email content at all.

Exam tip: remember that SMTP handles sending and relaying mail (including server-to-server transfer), while POP and IMAP handle retrieving mail for the end user; only SMTP fits a question about communication between email servers.