How Email Works

When you strip away all the postal metaphors, electronic mail (email) is just plain text sent around a network or between networks. That text is sent from server to server using Simple Mail Transport Protocol (SMTP).

Extensions to this protocol allow for authentication and posting error messages. Mail Transport (or Transfer) Agents (MTAs) work unseen by the user to manage sending and receiving.

There are two sets of rules for receiving mail. If you're getting mail from your Internet Service Provider (ISP), chances are you're using the Post Office Protocol, version 3 (POP3). This protocol sets up rules for downloading mail from a central server (your ISP) to a local client (your hard drive). The POP3 server then deletes the downloaded mail.

The other method for receiving mail is the Internet Mail Access Protocol (IMAP), version 4 generally seen in a corporate environment with users who need email access on the road or via multiple machines. With IMAP, mail is permanently stored on the central server, and users view their mail when they log in to the network.

Somewhat simplified, the following is what happens when you write an email message to [e-mail]. In the few seconds after you press the Send button in your email client

1. The client sends your text message up to the SMTP server.

2. The server's MTA queries a DNS server to locate the IP address for email.net and queues it for delivery with any other messages that may have come in at the same time.

3. Your SMTP server contacts the MTA at email.net on port 25. When the email.net server acknowledges the contact, your SMTP server sends your message, along with any others directed at email.net. The MTA for the (POP3 or IMAP4) server at email.net checks its list of users to make sure the occupant is there, acknowledges receipt of your message, and closes the connection.

4. The email.net MTA places your message in Occupant's Inbox. If configured that way, Occupant will be notified she has new mail when she logs in.

One of the best things about SMTP is that safeguards are in place so that few messages get lost in the ether. Should you accidentally send your message to [e-mail], the receiving server will send back a No such user error message, and maybe you'll notice your typo before resending. The same thing will happen if the SMTP server does not get an IP address for email.net from the DNS server in step 2. Also, it should be noted that email traffic is asynchronous, which means that if some link in the chain of computers is down and not functional, the email will queue up in the server and wait for the connection to be reestablished, only then sending it out to the intended recipients.

Similarly, if the receiving server never acknowledges your SMTP server's contact in step 3, the MTA will notify you that the receiving server is not responding. Depending on the MTA, it may tell you it will try again at another time. If the receiving server still doesn't respond, your MTA will notify you when it has given up; the message is undeliverable.

Caution

Although you can set your mail client to instruct the recipient to confirm receipt of every message you send, don't do this. It's annoying to the recipient. Use this tool sparingly for your most important messages.