email-how-it-works-2

When Anita composes a message, HAPPY NEW YEAR,

using her mail user agent (MUA), she types in, or selects from an address book, the e-mail address of her correspondent. She hits the "send" button.

1. Her MUA formats the message in Internet e-mail format and uses the Simple Mail Transfer Protocol (SMTP) to send the message to the local mail transfer agent (MTA), in this case smtp.a.org, run by Anita's Internet Service Provider (ISP).
2. The MTA looks at the destination address provided in the SMTP protocol (not from the message header), in this case adheesh@a.org. An Internet e-mail address is a string of the form localpart@exampledomain.com, which is known as a Fully Qualified Domain Address (FQDA). The part before the @ sign is the local part of the address, often the username of the recipient, and the part after the @ sign is a domain name. The MTA looks up this domain name in the Domain Name System to find the mail exchange servers accepting messages for that domain.
3. The DNS server for the b.org domain, ns.b.org, responds with an MX record listing the mail exchange servers for that domain, in this case mx.b.org, a server run by Adheesh's ISP.
4. smtp.a.org sends the message to mx.b.org using SMTP, which delivers it to the mailbox of the user Adheesh.
5. Adheesh presses the "get mail" button in his MUA, which picks up the message using the Post Office Protocol (POP3).

This sequence of events applies to the majority of e-mail users.
However, there are many alternative possibilities and complications to
the e-mail system:

• Anita or Adheesh may use a client connected to a corporate e-mail system, such as IBM Lotus Notes or Microsoft Exchange.
  These systems often have their own internal e-mail format and their
  clients typically communicate with the e-mail server using a
  vendor-specific, proprietary protocol. The server sends or receives
  e-mail via the Internet through the product's Internet mail gateway
  which also does any necessary reformatting. If Anita and Adheesh work for
  the same company, the entire transaction may happen completely within a
  single corporate e-mail system.
• Anita may not have a MUA on her computer but instead may connect to a webmail service.
• Anita's computer may run its own MTA, so avoiding the transfer at step 1.
• Adheesh may pick up his e-mail in many ways, for example using the Internet Message Access Protocol, by logging into mx.b.org and reading it directly, or by using a webmail service.
• Domains usually have several mail exchange servers so that they can
  continue to accept mail when the main mail exchange server is not
  available.
• E-mail messages are not secure if e-mail encryption is not used correctly.

It used to be the case that many MTAs would accept messages for any
recipient on the Internet and do their best to deliver them. Such MTAs
are called open mail relays.
This was important in the early days of the Internet when network
connections were unreliable. If an MTA couldn't reach the destination,
it could at least deliver it to a relay that was closer to the
destination. The relay would have a better chance of delivering the
message at a later time. However, this mechanism proved to be
exploitable by people sending unsolicited bulk e-mail
and as a consequence very few modern MTAs are open mail relays, and
many MTAs will not accept messages from open mail relays because such
messages are very likely to be spam.

Note that the people, e-mail addresses and domain names in this explanation are fictional: see Anita and Adheesh.
On Fast Track
with
Dr. Ashok Koparday