How it works:
In order to fully understand why email is here to stay we need to take a look at the history and the internals of the communication medium. The purpose of email as stated in the RFC (request for comment) document that specifies the technology and the protocols. RFC 821 written in 1982 states:
The objective of Simple Mail Transfer Protocol (SMTP) is to transferSeveral things are important about this statement, first note "simple" is in the name, when designed it was more important that messages were delivered (at some time) than that they arrived on a specific schedule. In many ways this makes it an electronic analog of real mail systems which more or less don't guarantee arrival time (unless you pay for that of course). The next and possibly most important aspect of this description that gives email the widespread use it enjoys today is "independent of the particular transmission subsystem" this means that different networks can mediate email. In the early 80's there were many transmission subsystems in use, ethernet was not the only connection method to network computers and a communication system that spanned the systems of the time was required, email had to work across all those systems in a agnostic manner. The solution was realized by using the tying the communication protocol to the transfer protocols that sat on top of the various types of networks in use, TCP (transfer control protocol)/IP (internet protocol) and its suite of ports or "sockets". SMTP was given the port 23 for exclusive use into machines for the purpose of mediate mail messages. The dual service of routing received messages to users was called POP (post office protocol) and was originally separate from SMTP, today most mail servers host both the message transfer function SMTP and the local mail delivery function POP. The important point of these protocols is that they are low level protocols, meaning they are part of the actual communication protocols of the internet itself as such any computer on the internet can enable applications to perform the respective functions by opening those ports and connecting to other computers. Now, having an SMTP server on a network allows that network to receive and forward messages but only by being aware of other SMTP servers on other networks, this forms a hiearchy of servers that create the global email system. Networks opt into this global system by registering a particular SMTP server on an internal network with an outside facing DNS (Domain Name Server) computer. DNS is the global address book of the entire internet, it is a hierarchical set of yellow pages for all the computers on the web that allows any data to move from a source computer in one network to a destination computer in another network. The data could be an smtp message, it could be video or audio, it could be video or audio embedded in an smtp message, DNS doesn't care...it's only purpose is to route the messages from source to destination. The DNS system is managed by ICANN (Internet Company for Assigned Names and Numbers) which leases out domain names to governments, educational institutions , corporations and individuals, because DNS is a public global service it is extremely robust to outages, data is routed around nodes that fail to respond to a forward request..without DNS computers would otherwise have to know the specific IP address of the destination computer to send data to it, by offloading that computation to a resident set of DNS servers that keep a list or index of names to numbers (IP's) this very simple but often repeated function is pushed to the network in a sense. It is this great power that the email system piggy backs on and that ensures that email as global reach.
mail reliably and efficiently.
SMTP is independent of the particular transmission subsystem and
requires only a reliable ordered data stream channel.
When you open a new yahoo email account, you don't have to worry about being able to send mail to your colleagues in your company mail system (so long as that system is connected to the wider web via DNS and smtp) , you don't have to worry about sending or receiving mail from your family on their various cell phone or broadband server provider email addresses. The existing network of public DNS servers and their associated SMTP routing hosts ensure that messages arrive at the destination reliably.
How it has grown:
The SMTP based email system has changed since it's description in 1982, back then SMTP only allowed simple text messages without any graphics or html (the language that web pages are written in that browsers an interpret) SMTP predates the www (world wide web) by 9 years so it was designed for much simpler messaging requirements. Today , smtp messages can contain video and audio and html and can contain foreign characters outside of the characters used in the roman set known in the pc world as ASCII text. The addition of MIME types to describe these more complex data types and send them as messages without reducing the robust nature of the service is a testament to how reliable it really is as a message delivery platform.
The new guard:
The emergence of the real time web has provided new options for message delivery. IM a messaging system for real time communication that is nearly 20 years old has now moved from stand alone installed clients running on computers to web based clients that can be accessed using a browser on a computer, laptop or smart phone. IM enables real time communication but because it is real time it is subject to the hiccups that email was designed to route around since delivery time was never meant to be guaranteed with SMTP. So IM servers a perpetually different niche for communication purposes than email. Another new technology is the micro-blogging methods used by companies like Twitter, the short form 140 character tweets enable bits of data to go out to whoever wishes to subscribe to a senders message stream by "following" them. This enables message delivery without requiring formal registration to an SMTP server (only sign up to the twitter service) but allows participants to gather large collections of followers to which they can send /receive messages en mass in a way that is anonymous (you don't necessarily know who your message is going to, with email you explicitly must state your list of recipients)
What is different:
The main difference between the email system and these other forms of messaging stands out quite clearly when one tries to send an IM to some one who isn't on the same IM network, or tries to send a short form message to some one who isn't on Twitter. These later forms of messaging are locked in the silos of the companies that provide the services, there is no equivalent of a global DNS network that ensures routing of messages across IM or "short form" networks. A simulation of this functionality as been arrived at by converting the messages into an open communication form called XML that allows different networks to capture and convert messages between networks but this doesn't guarantee the richness of the respective systems is conveyed in the message stream (for example connecting two users on IM from MSN messenger to AOL Instant Messenger via XML may allow the message and formatting to transit but may fail on transfer of a file through that message exchange) unlike SMTP where the protocol of communication is agreed upon by all makers of email servers , IM servers enable additional functionality that may not cross over to other IM networks. This prevents IM from attaining the level of reliability that email enjoys. Also, IM doesn't enable a method to route messages meant for other IM services or networks through an intermediate system or systems as email does this significantly reduces the scope and robustness of IM services and lock in the service usefulness to a particular IM implementation.
Additionally, IM was not made to compete with email on this front but to compliment it as stated earlier, for delivery of messages that are not time critical the more robust email system will always have a niche. The newest messaging idea is the short form messaging of twitter, however similar to IM it also is a specific protocol implementation that from implementation to implementation have different functions, currently it doesn't even have that problem as twitter is the only major provider of a network for short form messaging, again addressing a communication use case that is distinct from email.
Email satisfies a niche of communication that doesn't require delivery time but does require delivery. It is integrated into the backbone of the internet itself and predates the www, this integration gives it great robustness to failures in networks. The ability to route messages to other email servers through the main servers gives it a supreme advantage over IM and short form messaging by giving it a global scope. The additions to the service to enable delivery of rich messages containing html and audio or video media allow it in fact to serve as a more static version of the more real time messaging systems of IM and short form messaging making it the only option for such functions when it is infeasible or impossible to use the other protocols to perform the functions. As the internet advances the need for a queued non real time messaging system like email will not go away, in fact it will become more useful as a system for storing notifications and events that must be received at some time and that can be used as a global back up to similar functionality provided by independent IM and short form messaging networks.
http://www.faqs.org/rfcs/rfc821.html (The first RFC for SMTP the heart of email)
http://www.faqs.org/rfcs/rfc1939.html (pop3 the latest post office protocol for retrieving local mail from a mail server)
http://www.lemis.com/email/email-rfc.html (a list of email related RFC's)