Explanation:

The **Simple Mail Transfer Protocol (SMTP)** is the standard communication protocol used for sending and relaying electronic mail (email) between mail servers. When an email is sent, it typically travels from the sender's client to their mail server, and then to the recipient's mail server, all via SMTP. * **Option A (SMTP):** This is the correct answer. SMTP is fundamental for email transmission. * **Option B (FTP):** FTP (File Transfer Protocol) is used for transferring files between computers, not for email. * **Option C (TELNET):** TELNET is a network protocol used for remote command-line access to servers, not for email. * **Option D (SNMP):** SNMP (Simple Network Management Protocol) is used for monitoring and managing network devices, not for email communication.