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1 Structure of Email Services

Electronic mail is a very well-named messaging system. In many ways, it resembles the traditional postal mail services which it has largely replaced in everyday communication.In general, an individual email message is comprised of three parts: An “envelope”, a “header”, and a “message.” The envelope communicates delivery parameters. The headers include information on, at bare minimum, sender, recipient, subject, sending time stamp, and receiving time stamps of all mail servers on the path to the end user. Lastly, the mail message contains the actual information to be communicated, including any attached encoded files.

1.1 Basic Infrastructure

There are three protocols typically used in any standard email system: Simple Mail Transfer Protocol (SMTP) is used to send messages, and Post Office Protocol version 3 (POP3) or Internet Message Access Protocol version 4 (IMAP4) is used to receive messages.
The basic structure of an email transaction looks like this: Sender User Agent  ▷  Mail Transfer Agent(s)  ▷  Mail Delivery Agent  ▷  Recipient User Agent

1.2 Mail Transfer Agent

A Mail Transfer Agent is a piece of software that acts as a relay for email. It receives mail messages from a user’s mail client or another MTA, then stores it temporarily while attempting to pass it to either another MTA or an MDA. In the analogy of physical mail, an MTA is like a large post office, a temporary warehousing place for mail that has yet to reach its destination.

1.3 Mail Delivery Agent

Mail Delivery Agents are, to use the physical-mail analogy again, like a regional post office. They are the end point of a mail message’s journey across a network. Mail stays with the MDA until a user retrieves the mail through their client software.

1.4 Unsolicited Bulk Message Prevention

The comparisons between physical mail and email are never more apt than they are here. Both systems are practically clogged with junk - Advertisements, unsolicited requests for money, meaningless scrambled messages sent in some other strange attempt at getting rich quickly. An unsolicited bulk message filter is typically installed on the last leg of an email’s journey - Right between the MDA and the user. Of course, none of these systems are perfect, and as such numerous methods for training filters have come about.

1.5 Virus Control

Viruses, worms, trojan horses and other such malware are always loose in email messages, often going hand-in-hand with the junk mail problem. Virus filters sit next to UBM filters and try to analyze message attachments to determine if they contain any form of harmful software and if so, to clean it out or delete the mail entirely before harm can be done.
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