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.
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.
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.
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.
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.
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.
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.
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.
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.
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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