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Email Server 101
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Part 1: Email delivery via SMTP

When you send email message using your email program, such as Outlook Express, the message never goes to the recipient's computer directly. Moreover, your email program even does not know as to where on the net the recipient's computer is being located. All it needs to know is where to find a Mail Transfer Agent (MTA), which would accept the email message for delivery. This is very much like it happens in the real world: when sending mail from New York, NY to Sacramento, CA you do not need to bring up a roadmap. All you need to know is where is the nearest postal office, which would accept your package for delivery.

While you may speak to postal office in plain English, your email program will have to stick to the Simple Mail Transfer Protocol (SMTP) dialect. The protocol allows the email client to connect to the SMTP server, tell it recipient's email address and to transfer the message. After the transmission completed, your email program reports that the message has been sent. But as you can see now, being sent does mean neither being delivered, nor being received.

Anyway, all your email program needs in order to conduct the transaction is the SMTP server address (either IP number like 66.216.95.41 or name like smtp.myisp.com). The very moment you have entered the valid server info into the email account setup form, you can send mail.

Now back to the delivery process. Your SMTP server has to find another SMTP server, the one which does email processing for recipient's domain. For example, if the message was addressed to john@company.com, your server will be looking for the host, which is designated as a primary mail exchange for the company.com. This info (the MX record) is available from a nearby DNS server. After obtaining the address, SMTP servers connect to appropriate peer and transfer the message using the same procedure your email program did.

Now the message has been delivered. Technically speaking, the message has been appended to the file, which is considered to be the mailbox of user 'john' at the 'company.com' domain. This is not necessary mean the message was received by the recipient. But MTA agents (SMTP servers) did complete their job: the message has been delivered to the location, which is considered to be well known to the recipient. Speaking real life words for a moment again, the envelope has been delivered to the recipient's mailbox; Postal Service crewmen did their job. Now it is time for the recipient to check his or her mailbox. And this is exactly what the Post Office Protocol or POP is for.

...continued at the "Part 2. POP3 Server"

   
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