Kavi Mailing List Manager Help

Chapter 7. How Email Really Works

It's So Simple...or Is It?

From the user standpoint, email seems so simple. You set the email address of the person to whom you want to send the email, compose your message and click 'Send'.

All done.

In reality, sending your message off into the network cloud is a bit like sending Little Red Riding Hood into the deep dark woods. You never know what might happen.

Figure 7.1. How email really works

Diagram showing how email appears
	    to be a direct transfer from originating sender to
	    recipient, but actually involves multiple hops from
	    server to server within the network cloud.
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An Example

In this diagram, the sender is a human being using their company account to send an email to someone at a different company.

Step A: Sender creates and sends an email

The originating sender creates an email in their Mail User Agent (MUA) and clicks 'Send'. The MUA is the application the originating sender uses to compose and read email, such as Eudora, Outlook, etc.

Step B: Sender's MDA/MTA routes the email

The sender's MUA transfers the email to a Mail Delivery Agent (MDA). Frequently, the sender's MTA also handles the responsibilities of an MDA. Several of the most common MTAs do this, including sendmail and qmail (which Kavi uses).

The MDA/MTA accepts the email, then routes it to local mailboxes or forwards it if it isn't locally addressed.

In our diagram, an MDA forwards the email to an MTA and it enters the first of a series of "network clouds," labeled as a "Company Network" cloud.

Step C: Network Cloud

An email can encounter a network cloud within a large company or ISP, or the largest network cloud in existence: the Internet. The network cloud may encompass a multitude of mail servers, DNS servers, routers, lions, tigers, bears (wolves!) and other devices and services too numerous to mention. These are prone to be slow when processing an unusually heavy load, temporarily unable to receive an email when taken down for maintenance, and sometimes may not have identified themselves properly to the Internet through the Domain Name System (DNS) so that other MTAs in the network cloud are unable to deliver mail as addressed. These devices may be protected by firewalls, spam filters and malware detection software that may bounce or even delete an email. When an email is deleted by this kind of software, it tends to fail silently, so the sender is given no information about where or when the delivery failure occurred.

Email service providers and other companies that process a large volume of email often have their own, private network clouds. These organizations commonly have multiple mail servers, and route all email through a central gateway server (i.e., mail hub) that redistributes mail to whichever MTA is available. Email on these secondary MTAs must usually wait for the primary MTA (i.e., the designated host for that domain) to become available, at which time the secondary mail server will transfer its messages to the primary MTA.

Step D: Email Queue

The email in the diagram is addressed to someone at another company, so it enters an email queue with other outgoing email messages. If there is a high volume of mail in the queue—either because there are many messages or the messages are unusually large, or both—the message will be delayed in the queue until the MTA processes the messages ahead of it.

Step E: MTA to MTA Transfer

When transferring an email, the sending MTA handles all aspects of mail delivery until the message has been either accepted or rejected by the receiving MTA.

As the email clears the queue, it enters the Internet network cloud, where it is routed along a host-to-host chain of servers. Each MTA in the Internet network cloud needs to "stop and ask directions" from the Domain Name System (DNS) in order to identify the next MTA in the delivery chain. The exact route depends partly on server availability and mostly on which MTA can be found to accept email for the domain specified in the address. Most email takes a path that is dependent on server availability, so a pair of messages originating from the same host and addressed to the same receiving host could take different paths. These days, it's mostly spammers that specify any part of the path, deliberately routing their message through a series of relay servers in an attempt to obscure the true origin of the message.

To find the recipient's IP address and mailbox, the MTA must drill down through the Domain Name System (DNS), which consists of a set of servers distributed across the Internet. Beginning with the root nameservers at the top-level domain (.tld), then domain nameservers that handle requests for domains within that .tld, and eventually to nameservers that know about the local domain.

DNS resolution and transfer process

  • There are 13 root servers serving the top-level domains (e.g., .org, .com, .edu, .gov, .net, etc.). These root servers refer requests for a given domain to the root name servers that handle requests for that tld. In practice, this step is seldom necessary.

  • The MTA can bypass this step because it has already knows which domain name servers handle requests for these .tlds. It asks the appropriate DNS server which Mail Exchange (MX) servers have knowledge of the subdomain or local host in the email address. The DNS server responds with an MX record: a prioritized list of MX servers for this domain.

    An MX server is really an MTA wearing a different hat, just like a person who holds two jobs with different job titles (or three, if the MTA also handles the responsibilities of an MDA). To the DNS server, the server that accepts messages is an MX server. When is transferring messages, it is called an MTA.

  • The MTA contacts the MX servers on the MX record in order of priority until it finds the designated host for that address domain.

  • The sending MTA asks if the host accepts messages for the recipient's username at that domain (i.e., username@domain.tld) and transfers the message.

Step F: Firewalls, Spam and Virus Filters

The transfer process described in the last step is somewhat simplified. An email may be transferred to more than one MTA within a network cloud and is likely to be passed to at least one firewall before it reaches it's destination.

An email encountering a firewall may be tested by spam and virus filters before it is allowed to pass inside the firewall. These filters test to see if the message qualifies as spam or malware. If the message contains malware, the file is usually quarantined and the sender is notified. If the message is identified as spam, it will probably be deleted without notifying the sender.

Spam is difficult to detect because it can assume so many different forms, so spam filters test on a broad set of criteria and tend to misclassify a significant number of messages as spam, particularly messages from mailing lists. When an email from a list or other automated source seems to have vanished somewhere in the network cloud, the culprit is usually a spam filter at the receiver's ISP or company. This explained in greater detail in Virus Scanning and Spam Blocking.


In the diagram, the email makes it past the hazards of the spam trap...er...filter, and is accepted for delivery by the receiver's MTA. The MTA calls a local MDA to deliver the mail to the correct mailbox, where it will sit until it is retrieved by the recipient's MUA.


Documents that define email standards are called "Request For Comments (RFCs)", and are available on the Internet through the Internet Engineering Task Force (IETF) website. There are many RFCs and they form a somewhat complex, interlocking set of standards, but they are a font of information for anyone interested in gaining a deeper understanding of email.

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It's Like Regular Mail

Email construction and delivery is similar to regular mail by design, because email is modeled on regular mail.

A Message Enclosed in an Envelope

An email message is constructed like a letter you'd send through the postal service: a message enclosed in an envelope. The email envelope header is analogous to the envelope of a hardcopy letter, but some of the information that is ordinarily present on a hardcopy envelope is contained in the message header instead of the envelope header. This header header also contains information that is not usually found on a real-world envelope, but is essential to email delivery and troubleshooting. The envelope header is usually hidden when you view an email, and the message header is usually visible. Together, these two headers are called the full header.

Message Header Fields

Anyone who has used email is familiar with the message header, which is displayed when you view an email message and includes the 'From:', 'To:', 'Cc:', 'Date:' and 'Subject:' fields. The content of these fields differs only slightly from regular mail, because the 'From:', 'To:' and 'Cc:', fields in an email identify the sender and intended recipients by email address.

The Date Field

The message header's 'Date:' field is applied by the originating sender's MUA, so it is only as accurate as the clock on the sender's computer.

The Subject: Field

The 'Subject:' field isn't used in regular mail except in formal business letters where its closest analogy is the 'Re:' line, but this field is necessary for email because without it, you could only differentiate one email from another in the inbox based on the 'From:', 'To:' and 'Date:' fields.

The Return-Path

Email contains more detailed information about its delivery process than the single postmark of regular mail. As the email passes through the delivery chain, MTAs add more interesting and reliable postmark-like timestamps and MTA location information, including the envelope header's 'Received' fields (described in the next section) and the 'Return-Path', which contains the identity of the sender, such as <username@domain.tld>.

The 'Return-Path' is often referred to as the "envelope sender", and this is the address that mailing lists use to determine "who" sent a message. The 'Return-Path' is also the address to which bounces are sent.

The Received field

A 'Received' field added by each MTA in the email delivery chain as it accepts a message for transfer. When a receiving MTA accepts the email for relay or local delivery, it records information about the transaction in the email's envelope header. This includes a message ID that it uses, and which will appear in the MTA server logs, timestamps indicating the time of the transfer and the identity of the sending MTA. If you follow the 'Received' entries in order, they will lead you back to the originating MTA (but not to the senders email address).

This information about the true identity of the sending MTA is valuable when troubleshooting issues with spam or malicious messages. These kinds of messages often contain forged identities in the 'From:' and 'Reply-To' fields, but the true identity of the sending MTA can be extracted from the envelope header. When contacted by the sending MTA, the receiving MTA checks to see whether the hostname provided to it by the originating MTA resolves to a unique IP address. If it does, then it is a fully qualified domain name and the receiving MTA adds the information to the 'Received' field. If the hostname does not resolve properly, the receiving MTA adds the originating MTA's IP address (and possibly also the true hostname) instead.

The Reply-To field

The envelope header also contains a 'Reply-To' address provided by the sender that the receiver can use to reply to the sender. This is analogous to the return address on regular mail. Email messages, particularly automated notifications and messages from mailing lists, often set a different 'Reply-To' address so that bounced messages will be sent to an automated bounce handler. Like a return address on regular mail, the 'Reply-To' address doesn't have to be a real address, but if it isn't, mail sent to it will be undeliverable. Spam and messages containing malware are likely to have false information in the 'From:' and 'Reply-to' fields, but the originator's true Internet address is recorded in the first 'Received' entry in the email's envelope header.

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Troubleshooting Email Issues

What if something goes wrong?

Messages eaten by spam filters fail silently because a sending MTA has already successfully completed the transfer to a firewall server. Message failures that occur when an MTA is attempting to transfer a message will not fail silently, so except when a message was unfortunate enough to have been mistaken for spam or is currently languishing in a moderation queue or waiting for the next archive update, users will know what happened to their message because MTAs reliably complain when transfers fail.

A sending MTA can encounter two general kinds of problems transferring an email: transient or permanent failures.

transient failures

If a transient error occurs, the MTA will hang onto the message, periodically retrying the delivery until it either succeeds or fails, or until the MTA decides that the transient issue is really a permanent condition.

permanent failures

If the MTA cannot deliver the message (it has received a fatal error message or failed to complete the transfer after repeated attempts), it bounces the message back to the sender. If the sender is a mailing list, the bounce may be handled by automated bounce-handling software. For more information, see Bounces and Automated Bounce Handling.

Links to troubleshooting materials

This documentation has extensive information on troubleshooting. See Introduction to Troubleshooting for a list of available materials.

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