Mail accross the internet or any network for that matter is pretty simple. There are a few entities but the concepts aren't that complex. Most of the complexities in the internet mail framework is keeping bad guys out.

There are three major entities that handle mail.

• MTA - Mail Transfer Agent
• Mail Store - Where the mail is kept (IMAP or POP)
• MUA - Mail User Agent - the software that gets the mail from the mail store.

The Mail Transfer Agent is what moves mail accross the network and the internet i.e. around the world. Most MTAs use the Simple Mail Transfer Protocol (SMTP) as outlined in RFC 2821

RFC 821 and 2821 are about the protocol to transmit mail over the wire. The Simple Mail Transfer Protocol (SMTP) which isn't so simple anymore.

The mail store is where the mail is kept. Also known as the mail box - but it's not a box.

Mail stores come in two flavors:

• IMAP - Internet Message Access Protocol RFC 3501
• POP - Post Office Protocol RFC 1939

Both protocols handle email however, they do it in different ways.

IMAP - maintians the mail on the mail store (server). The client then access and manages thier mail using the API's of the mail store. The major advantage of this protocol is the client can access their mail from any device using an MUA.

POP - mail is stored on the mail store until the client comes and gets it, at which time the mail is downloaded to the client and deleted from the store. There is no mail maintained on the mail store (server). Mail downloaded from a POP mail store must be accessed by whatever downloaded it. The major advantage to this protocol is the client can access mail offline.

Some popular open source mail stores:

• Courier IMAP
• Cyrus
• Dovecot

The Mail User Agent is the front end of any mail system. It is what gets the mail once it is tucked away in a mail box by a MTA, after its journey.

The original RFC to cover email back in the ARPAnet days was RFC 733, Standard for the Format of ARPA Network Text Messages - 1977. It was replaced by RFC 822 of the same title in 1982. email was called text messaging back then... look at the title of the RFCs. Of course, what we call text messaging or texting is a completely different story.

RFC 822 and 2822 deal with the format of a message transmitted over the wire.

The email system exploded since the old days with the advent of MIME content, etc. It grew so much that the original RFC 822 needed to address more... before it got out of hand. So the folks at IETF published a new guide to email, RFC 2822. Since the governemt folded up the ARPAnet which gave way to the internet, they just called it Internet Message Format. Published in 2001.