Published on 07 October 2011


Email delivery is analogous to the post office system.  Email originates from a sender, who must deliver it to a post office (a mail server).  Once in the post office, it is up to the postal system to figure out where the message is to be delivered.  Once it arrives at the post office responsible for a mailbox, the message is delivered into the mailbox.  It is up to the recipient to go to that post office to retrieve the messages stored in the mailbox. Thus, there are two underlying functions: getting email to and from the post office, and the post office itself that manages the task of receiving the messages and transporting to the destination.

The Post Office

The method/protocol used to send mail to a server, or from server to server is called SMTP (Simple Mail Transfer Protocol).  SMTP defines the method of addressing (the envelope), but does not care about the content.  The post office consists of servers that receive the mail from users, then process the mail, figure out where it should be delivered, and finally place it into the recipient’s mailbox.  The processing part can take a number of steps.  For example, the server receives a single message, regardless of the number of recipients.  The server needs to determine how many recipients are listed for each message and generate one copy for each recipient, then determine which server or servers handle the mail for each of the recipients and pass on the message(s).  Often there are multiple servers involved, where one server or bank of servers are designated to receiving incoming mail, then pass the message on to other servers dedicated to sending the messages out.  Other times, intermediary servers process the messages and strip out viruses and spam first before routing the messages on.  The list of servers that touched each message can be viewed in the message header.

The User Experience

To retrieve messages from the mailbox, several methods/protocols are available.  The following are the most widely used types of email retrieval methods (in no particular order):

1. POP3


3. Webmail

4. Gmail

5. MicroSoft Exchange

6. Mobile

7. Unified Messaging

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