Internet Email
The Internet mail system can be easily conceptualized by introducing the key elements. A message is a piece of information that one Internet user wishes to send to another. It may include multiple parts, including binary files, that may be attached to the message. A message is sent from an MUA; MUAs are also used to read messages that are received from others.
Note
The Internet mail system uses the Multipart Internet Mail Extensions (MIME) to attach files to mail messages. MIME is a series of specifications that describe how to represent binary data as text so that they may be sent via text-based electronic mail. The combination of the SMTP protocol and the MIME specification is the basis of the modern Internet mail system and is often called SMTP/MIME.
The most important elements of the Internet mail system are:
- Mail user agent (MUA)
A client program used by a user to send or receive email. An MUA could also be a program or script that emulates the behavior of a typical MUA by sending or receiving email.
- Mail transfer agent (MTA)
A server program that transfers email from one machine on the Internet to another—a mail server.
- Mail delivery agent (MDA)
A small program used by an MTA to write a message into a user’s mailbox.
- Mail retrieval agent (MRA)
A service that retrieves messages from a mailbox on a remote server to a user’s MUA.
For example, an email message is sent by an MUA to an MTA. This is known as the sending MTA. The sending MTA transfers the message to one or more MTAs until the message reaches its destination, at a receiving MTA. The receiving MTA actually holds an account for the recipient of the message. The receiving MTA transfers the message to an MDA. The MDA writes the message to the recipient’s mailbox, normally a file on a filesystem. The recipient gets the message from the mailbox by using another mail user agent to read the message. MUAs are also used to sort, order, and sometimes store mail messages between sessions.
When the recipient uses a mail server that is not his local machine, the mailbox has to be read at a distance. This is done by what I will call a mail retrieval agent (MRA). This is not a standard term but describes a useful concept.
MUAs send mail to MTAs using the Internet’s Simple Mail Transfer Protocol (SMTP), possibly with MIME extensions for binary attachments. MTAs also use SMTP among themselves. MRAs commonly use the Internet’s Post Office Protocol (POP), the more recent Internet Message Access Protocol (IMAP), or the much older Unix to Unix Copy (UUCP) Protocol. UUCP is not covered in this book.