|Here you'll find
most of the systems and services used to move an email from one place to
You never see or directly use the protocols that move your emails all over
the internet, but all email on internet and in many local networks use
smtp services when handling email.
The goal is to explain as much as possible, in a very easy to understand
Email in your computer
A mail-box is a file, or possibly a directory of files, where incoming
messages are stored.
A mail user agent, or MUA, is an application run directly by a user. User
agents are used to compose and send out-going messages as well as to
display, file and print messages which have arrived in a user's
mail-box. Examples of user agents are Eudora, Outlook Express and Outlook.
Mail transfer agents (MTAs) are used to transfer messages between machines.
User agents give the message to the transfer agent, who may pass it
onto another transfer agent, or possibly many other transfer agents.
Users may give messages to transfer agents directly, but this requires some
expertise on the part of the user and is only recommended for experts.
Transfer agents are responsible for properly routing messages to their
destination. While their function is hidden from the average user,
theirs is by far the most complex part of getting messages from their source
to their destination. The most common transfer agent is sendmail. The
protocol used when connecting to an mail
server, is called SMTP.
Pop Account, Mailing Lists and Aliases
The place where your email is stored in a server is called pop account (and
by many mailbox).
The most common used protocol (language) when speaking with a pop account is
POP (Post Office Protocol).
Another more advanced protocol is IMAP.
A mail address is the address to a pop account, alias or mailinglist.
A mail address consists of some text followed by an @ and then followed by
The first part (to the left of @) is the pop account (mailbox) or alias
followed by @ and finally the domain (ex. blueadmiral.com).
A mailinglist is a list with a name, containing one or more mail addresses,
mailinglist addresses or aliases.
Is a mail address that send incoming email along to another email address
Headers & Bodies
Each message consists of two parts. The headers contain information about
who authored the message, the intended recipients, the time of
creation, the subject of the message, delivery stamps, ... Each header is of
the form "keyword: value", where keyword is a special word (like From
or Date) identifying the type of information contained in that header, and
value is the information itself.
A blank line always separates the headers from the body. The body contains
the information the sender is trying to communicate. The "message" as
most people think of it is really the body of the message.
For many years, most messages were plain text in the US-ASCII character set,
so no structure was needed for message bodies. The explosion of
messaging in Europe and Asia in the mid 1990s and that of transmission of
multi-media messages in the late 1990s brought about such a need.
Content-Type: text/plain; charset=us-ascii
indicates that the message consists of plain text in the US-ASCII character
set. MIME also specifies how to encode data when necessary (more on
this below). It is the responsibility of the receiving user agent to use
this information to display the message in a form that will be
understood by the user.
The language spoken between transfer agents is known as a transfer protocol.
There are many in existence; the most common is Simple Mail Transfer
Envelopes and Bodies
SMTP uses the concept of an envelope to transfer messages; this merely
contains information about from whom the message originated and to
whom it is destined. The originator address is important: in case there is a
problem transferring or delivering the message, the originator can be
The SMTP body is the entire message as defined above in Headers & Bodies. So
the message headers plus the message body equals the SMTP body. The
term SMTP body is not used that commonly, but it is important to distinguish
it from the message body.