[1][2][3] Simple Mail Transfer Protocol (SMTP) is an Internet standard for electronic mail (e-mail) transmission across Internet Protocol (IP) networks. SMTP is a text-based protocol, in which a mail sender communicates with a mail receiver by issuing command strings and supplying necessary data over a reliable ordered data stream channel, typically a Transmission Control Protocol (TCP) connection.
[4][5] The Post Office Protocol (POP) is an application-layer Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection. Like many other older Internet protocols, POP originally supported only an unencrypted login mechanism. Although plain text transmission of passwords in POP still commonly occurs, POP3 currently supports several authentication methods to provide varying levels of protection against illegitimate access to a user's e-mail.
[6][7] The Internet Message Access Protocol (IMAP) is one of the two most prevalent Internet standard protocols for e-mail retrieval. IMAP supports both on-line and off-line modes of operation. E-mail clients using IMAP generally leave messages on the server until the user explicitly deletes them. This and other characteristics of IMAP operation allow multiple clients to manage the same mailbox. Most e-mail clients support IMAP in addition to POP to retrieve messages; however, fewer Internet service providers (ISPs) support IMAP. IMAP offers access to the mail store. Clients may store local copies of the messages, but these are considered to be a temporary cache.
[8][9] File Transfer Protocol (FTP) is a standard network protocol used to exchange and manipulate files over a TCP/IP-based network, such as the Internet. FTP is built on a client-server architecture and utilizes separate control and data connections between the client and server applications. FTP is used with user-based password authentication or with anonymous user access.
[10][11][12] The SSH File Transfer Protocol (sometimes called Secure File Transfer Protocol or SFTP) is a network protocol that provides file access, file transfer, and file management functionality over any reliable data stream. This protocol assumes that it is run over a secure channel, such as SSH, that the server has already authenticated the client, and that the identity of the client user is available to the protocol.