Which application-layer protocol is used to send email messages from a client to a server? 

Simple Mail Transfer Protocol (SMTP) is the protocol used to send emails from clients to mail servers and between mail servers for relaying messages. It works at the application layer and is a fundamental part of the email communication system. When an email is sent, SMTP handles the task of transferring the email to the appropriate server, ensuring it reaches its destination. It typically operates on port 25 or 587. SMTP only handles the sending of emails, while protocols like POP3 and IMAP are used for receiving emails. Why other options are wrong: a) POP3 (Post Office Protocol 3) : Used for retrieving email from a server but not for sending emails. b) IMAP (Internet Message Access Protocol) : Another protocol for retrieving emails but not for sending them. c) FTP (File Transfer Protocol) : Used for transferring files between systems, not for email transmission. e) SNMP (Simple Network Management Protocol) : Used for network management, not for email transmission.