The explanation of how it all works is something that has been written many times. You should do some research via search engines because nobody can give you a comprehensive answer in a few forum posts.

In the beginning there were no email servers or clients. People had no PCs, only mainframe and minicomputers where an individual had access through batch terminals based on punched cards or paper tape and printers and in rarer cases through CRT terminals and teleprinters. But even then there were email systems of a sort. As terminals became more common these became more sophisticated, handling longer more letter-like messages.

Soon more of these computers were connected by point to point network links based on proprietary protocols. People started to allow email systems to forward email to each other. As the early Internet began, and even before TCP/IP existed as we know it now, an early version of SMTP was developed. This was a computer to computer mail forwarding protocol and was often used along with the older protocols to move mail from system to system.

Only much later as the Internet developed and PCs and Workstations became available to those with a lot of money did we begin to see something like email as the average person knows it now. The Internet was still not public, but those in the inner circle with access to it and even those with access to networked desktop computers began using early "client/server" email. In some cases there still wasn't any TCP/IP used and no SMTP or other protocols we associate with email today used either.

Gradually TCP/IP became more common and early POP and SMTP protocols began being the standards for client to server email transmission. SMTP still was (and is) only for sending an email or bundle of emails from machine to machine. DNS was enhanced with support for MX (mail exchange) records to assist in the development of relay server networks using SMTP to move email around. IMAP came along as a POP alternative, and a few other mail retrieval protocols were developed but most never gained momentum.

PCs finally became cheaper and more common and soon the Internet went public. This is where the average Joe started to come along, and by then everything was pretty much set in concrete. Little has evolved further except for somewhat better security for the SMTP network and more advanced algorithms for routing and relaying mail across the SMTP backbone network.

Email is so high in volume now that many large organizations have to mimic the SMTP/DNS backbone network in house with sub-backbone networks to help keep the traffic away from other business.


Today any small-scale SMTP server or large-scale in house email backbone network get access to the global backbone via SMTP relay service providers. Some ISPs offer this kind of service to commercial customers but seldom to residential customers. You'd need to contract with a service provider specializing in email gateway services.


Look at things like:

http://en.wikipedia.org/wiki/Mail_submission_agent

http://dev.mutt.org/trac/wiki/MailConcept

http://www.ida.liu.se/~TDDI09/lectures/TDDI09-F4.pdf

... to get an idea of how things fit together.