Electronic mail (email) is one of the oldest and also one of the most widely used Internet services. Mail communication is performed by many application protocols (SMTP, POP3, IMAP4, etc.), and this chapter describes these protocols.
Support from Operating Systems
At present, email is implemented probably in all existing operating systems. Logically, operating systems of end-user stations implement clients of particular protocols while server operating systems implement mail servers. As was described in the case of TCP/IP protocol, a mail server is far beyond the client-server architecture. It is better to use the more appropriate term Mail Transfer Agent (MTA) instead of mail server.
Two types of security are used for email: ...