SMTP, or Simple Mail Transfer Protocol, functions on the application layer and is made up of four different programs that, altogether, allow mail to be sent across the web.

MUA, or Mail Users Agent, is a SMTP program that is used when typing an e-mail, and often includes some sort of editor. From there, the mail is sent to the MTA, or Message Transfer Agent. This program takes the mail and passes it from the sender to the receiver. The mail is then given to the LDA, or Local Delivery Agent, which receives the email on the recipient’s computer.

Finally, the Mail Notifier does exactly what it sounds like—it tells the recipient that they’ve got mail. Programs outside of this process that are still included in the protocol are the Directory Services and Post Office. The Directory Services include the list of users on a system, while the Post Office is where the mail is stored.

An email message is made up of three basic parts; the envelope, headers, and body. The envelope utilizes the MAIL and RCPT commands to include the addresses of both the sender and the recipient. Headers always have names, which are followed by a colon and their value. A common format is From, Date, Reply-To, received Message-ID, To, and Subject. The body is simply the contents of the message, sent in 7 bit ASCII code.

There are, of course, other Mail Protocols. POP3, or Post Office Protocol version 3, that works on 7^th layer of OSI Layer Model retrieves mail from internet mail servers by direction of users. IMAP4, or Internet Mail Access Protocol version 4, is simply an updated version of POP3. It has now completely replaced it.

MIME, or Multipurpose Internet Mail Extension, is what defines a file’s attachment to SMTP messages. X.400 is the International Telecommunication Union standard that defines the transfer protocols for sending mail. MHS, or the Message Handling Service, is a Novell program used on Netware networks for mail services. Directory services include LDAP, or Lightweight Directory Access Protocol, and X.500 organize and share objects on large networks. Email can also be included in application programs by using APIs, or Application Programming Interfaces. Microsoft’s office products support mail on the application level through the usage of MAPI, or Microsoft’s Messaging API. All of those separate from Microsoft use VIM, or Vendor-Independent Messaging, which was created by Lotus.

This was a short overview on the Email protocols. We would love to know your thoughts on this in the comment section below.