Explanation
SMTP (Simple Mail Transfer Protocol) is the primary protocol used for sending and receiving electronic mail (email) on the Internet. It is responsible for the transmission of emails between mail servers. SMTP works in conjunction with other protocols such as POP3 (Post Office Protocol version 3) and IMAP (Internet Message Access Protocol), which are used by email clients to retrieve messages from a mail server.
-Telnet is a protocol used for remote terminal access.
-FTP is the File Transfer Protocol, which is used for transferring files between computers on a network.
-RDP stands for Remote Desktop Protocol, and it is used for remote desktop access and control.
So, for electronic mail specifically, SMTP is the relevant protocol.