Which protocol is used to send email?

Simple Mail Transfer Protocol (SMTP) is a protocol for sending e-mail messages between servers. POP is a protocol used to retrieve e-mail from a mail server. NFS is a TCP/IP client/server application developed by Sun that enables different types of file systems to interoperate regardless of operating system or network architecture. FTP is the protocol that is used to facilitate file transfer between two machines.

Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, 2001, John Wiley & Sons, Page 88.