Which of the following protocols would be used to send email to an email server?

NTP (Network Time Protocol) is used to synchronize the clocks of networked devices to a time source and has no role in email delivery.

FTP (File Transfer Protocol) transfers files between hosts over TCP ports 20 and 21 and is not used for email transmission.

SNMP (Simple Network Management Protocol) is used for monitoring and managing network devices such as routers and switches, not for sending email.

SMTP is the standard application-layer protocol for email submission and relay, operating on TCP port 25 (server-to-server) or port 587 (client submission). When a user sends an email, the mail client uses SMTP to deliver the message to the outgoing mail server, which then relays it to the recipient's mail server.