FTP works by opening two connections that link the computers trying to communicate with each other. One connection is designated for the commands and replies that get sent between the two clients, and the other channel handles the transfer of data. During an FTP transmission, there are four commands used by the computers, servers, or proxy servers that are communicating. These are “send,” “get,” “change directory,” and “transfer.”

While transferring files, FTP uses three different modes: block, stream, and compressed. The stream mode enables FTP to manage information in a string of data without any boundaries between them. The block mode separates the data into blocks, and in the compress mode, FTP uses an algorithm called the Lempel-Ziv to compress the data.

Source: https://www.fortinet.com/resources/cyberglossary/file-transfer-protocol-ftp-meaning

TCP is one of the basic standards that define the rules of the internet and is included within the standards defined by the Internet Engineering Task Force (IETF). It is one of the most commonly used protocols within digital network communications and ensures end-to-end data delivery.

Source: https://www.fortinet.com/resources/cyberglossary/tcp-ip

Clients usually retrieve all messages and then delete them from the server, but POP3 does allow retaining a copy on the server. Nearly all email servers and clients currently support POP3.

Source: https://developer.mozilla.org/en-US/docs/Glossary/POP

It also follows the client/server model. On one side, we have an IMAP client, which is a process running on a computer. On the other side, we have an IMAP server, which is also a process running on another computer. Both computers are connected through a network.

Source: https://www.javatpoint.com/imap-protocol