<p><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;"><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;">In this diagram, the sender is using their company account to send an email to someone at a different company.</span></span></span></p>
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<p style="text-align: center;" mce_style="text-align: center;"><span style="font-size: 10pt;" mce_style="font-size: 10pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;"><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><br /><img src="http://emailcenter.hypersupportonline.com/images/stories/howemailworks.png" mce_src="http://emailcenter.hypersupportonline.com/images/stories/howemailworks.png" alt="howemailworks" width="500" border="0" height="392"></span></span></span></p>
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<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;"><b>Step A:</b> Sender creates and sends an email</span></span></span></p>
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<p lang="en"><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;">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.</span></span></span></p>
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<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;"><b>Step B:</b> Sender's MDA/MTA routes the email</span></span></span></p>
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<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;">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.</span></span></span></p>
<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;">The MDA/MTA accepts the email, then routes it to local mailboxes or forwards it if it isn't locally addressed.</span></span></span></p>
<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;">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.</span></span></span></p>
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<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;"><b>Step C:</b> Network cloud</span></span></span></p>
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<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;">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, 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.</span></span></span></p>
<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;">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.</span></span></span></p>
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<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;"><b>Step D:</b> Email queue</span></span></span></p>
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<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;">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.</span></span></span></p>
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<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;"><b>Step E:</b> MTA to MTA transfer</span></span></span></p>
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<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;">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.</span></span></span></p>
<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;">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.</span></span></span></p>
<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;">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.</span></span></span></p>
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<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;"><b>DNS resolution and transfer process</b></span></span></span></p>
<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;">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.</span></span></span></p>
<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;">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.</span></span></span></p>
<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;">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.</span></span></span></p>
<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;">The  MTA contacts the MX servers on the MX record in order of priority until  it finds the designated host for that address domain.</span></span></span></p>
<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;">The sending MTA asks if the host accepts messages for the recipient's username at that domain (i.e.,
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</span>) and transfers the message.</span></span></span></p>
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<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;"><b>Step F: </b>Firewalls, spam and virus filters</span></span></span></p>
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<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;">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.</span></span></span></p>
<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;">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.</span></span></span></p>
<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;">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.</span></span></span></p>
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<p><b><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;">Delivery</span></span></span></b></p>
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<p><span style="color: rgb(0, 0, 0);" mce_style="color: #000000;"><span style="font-size: 12pt;" mce_style="font-size: 12pt;"><span style="font-family: arial,helvetica,sans-serif;" mce_style="font-family: arial,helvetica,sans-serif;">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.</span></span></span></p>
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