Email - Operation Overview

Operation Overview

The diagram to the right shows a typical sequence of events that takes place when Alice composes a message using her mail user agent (MUA). She enters the email address of her correspondent, and hits the "send" button.

1. Her MUA formats the message in email format and uses the Submission Protocol (a profile of the Simple Mail Transfer Protocol (SMTP), see RFC 6409) to send the message to the local mail submission agent (MSA), in this case smtp.a.org, run by Alice's internet service provider (ISP).
2. The MSA looks at the destination address provided in the SMTP protocol (not from the message header), in this case bob@b.org. An Internet email address is a string of the form localpart@exampledomain. 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 or a fully qualified domain name. The MSA resolves a domain name to determine the fully qualified domain name of the mail exchange server in the Domain Name System (DNS).
3. The DNS server for the b.org domain, ns.b.org, responds with any MX records listing the mail exchange servers for that domain, in this case mx.b.org, a message transfer agent (MTA) server run by Bob's ISP.
4. smtp.a.org sends the message to mx.b.org using SMTP.

This server may need to forward the message to other MTAs before the message reaches the final message delivery agent (MDA).

1. The MDA delivers it to the mailbox of the user bob.
2. Bob presses the "get mail" button in his MUA, which picks up the message using either the Post Office Protocol (POP3) or the Internet Message Access Protocol (IMAP4).

That sequence of events applies to the majority of email users. However, there are many alternative possibilities and complications to the email system:

• Alice or Bob may use a client connected to a corporate email system, such as IBM Lotus Notes or Microsoft Exchange. These systems often have their own internal email format and their clients typically communicate with the email server using a vendor-specific, proprietary protocol. The server sends or receives email via the Internet through the product's Internet mail gateway which also does any necessary reformatting. If Alice and Bob work for the same company, the entire transaction may happen completely within a single corporate email system.
• Alice may not have a MUA on her computer but instead may connect to a webmail service.
• Alice's computer may run its own MTA, so avoiding the transfer at step 1.
• Bob may pick up his email in many ways, for example 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.
• Email messages are not secure if email encryption is not used correctly.

Many MTAs used to accept messages for any recipient on the Internet and do their best to deliver them. Such MTAs are called open mail relays. This was very 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 closer to the destination. The relay stood a better chance of delivering the message at a later time. However, this mechanism proved to be exploitable by people sending unsolicited bulk email and as a consequence very few modern MTAs are open mail relays, and many MTAs don't accept messages from open mail relays because such messages are very likely to be spam.