As an email specialist we often talk in acronyms and these are the three main ones we use on a daily basis:
SMTP: Simple Mail Transfer Protocol is a standard set of rules that allows electronic devices to transfer electronic mail (email) from one device to another. Mail servers and other message transfer agents use SMTP to send and receive mail messages.
Although some mail clients and apps may use non-standard protocols internally, all use SMTP when sending to or receiving email from outside their own systems.
For retrieving messages, IMAP and POP3 are standard, and we wont worry about some proprietary servers that implement their own protocols such as Exchange ActiveSync.
IMAP (Internet Message Access Protocol) is a standard email protocol that stores email messages on a mail server, but allows the end user to view and manipulate the messages as though they were stored locally on the end user’s computing device(s).
POP3 (Post Office Protocol 3) is a client/server protocol in which email is received and held for you by your Internet server.