An email server receives mail from another MTA, a mail user agent (MUA) or a mail submission agent (MSA) with the details of the transmission being defined by SMTP. When an MTA receives a mail and the recipient of the mail is not hosted locally, the mail is forwarded to another MTA. Every time this happens the MTA adds a "received" trace header on the top header of the message. This shows all the MTAs that have handled the message before it arrived in the recipient’s inbox. This useful feature allows administrators to see whether an optimal path was taken.
 

Post Office Protocol is the primary protocol behind email communication. POP works through a supporting email software client that integrates POP for connecting to the remote email server and downloading email messages to the recipient’s computer machine. 

POP uses the TCP/IP protocol stack for network connection and works with Simple Mail Transfer Protocol (SMTP) for end-to-end email communication, where POP pulls messages and SMTP pushes them to the server. As of 2012, Post Office Protocol is in its third version known as POP 3 and is commonly used in most email client/server communication architecture.
 

IMAP was originally designed as a remote mailbox protocol in 1986 by Mark Crispin. This was during the popular use of Post Office Protocol (POP). IMAP and POP are still both supported by the majority of modern email servers and clients. However, IMAP is a remote file server, while POP istores and forwards. In other words, with IMAP, all emails remain on the server until the client deletes them. IMAP also permits multiple clients to access and control the same mailbox.

When a user requests an email, it is routed through a central server. This keeps a storage document for the email files. Some of IMAP benefits include the ability to delete messages, search for keywords in the body of emails, create and manage multiple mailboxes or folders, and view the headings for easy visual scans of emails.

IMAP is still used extensively, but is less important now that so much email is sent via web-based interfaces such as gMail, Hotmail, Yahoo Mail, etc. 
 

SMTP is one of the most common and popular protocols for email communication over the Internet and it provides intermediary network services between the remote email provider or organizational email server and the local user accessing it.

SMTP is generally integrated within an email client application and is composed of four key components:

SMTP works by initiating a session between the user and server, whereas MTA and MDA provide domain searching and local delivery services.