Electronic mail | e-mail | Part 1
What Is Electronic mail or e-mail?
Electronic mail or e-mail (from ‘electronic mail’) is an Internet service from which the user can send or receive messages. It is the Internet application most known and used today. Its creation dates back to 1972, when Ray Tomlinson installed on the ARPANET system able to exchange messages between different universities, but who is really defined the operation was Jon Postel.
It is the counterpart of digital and electronic mail and hard copy. Unlike the latter, the delay comes from the sender to the recipient is usually a few seconds / minutes, although there are exceptions which delay the service until a few hours.
Service model
The aim of the e-mail message transfer from one user to another.
Each user can have one or more mailboxes on which it receives messages that are stored. When desired, the user can see the contents of his box, organize, and send messages to one or more users.
Access to the mail box is normally controlled by a password or other forms of authentication.
How to access the service is therefore asynchronous, or for transmission of a message sender and recipient must be simultaneously active or connected.
Delivery of messages sent to the recipient is not guaranteed. If an SMTP server fails to deliver a received message, normally try to send a notification to the sender to advise of non-delivery, but this notification is itself an e-mail (generated automatically by the server), and then his delivery is not guaranteed.
The sender may also request a delivery or read the messages sent, but the recipient is usually able to decide if he wants to or not to send such confirmation. The meaning of the read receipt can be ambiguous, because you receive a message for a few seconds in a client does not mean having read, understood, or have shared the content.
Each box is associated with one or more email addresses. These have the form username @ domain, where username is a name chosen by the user or administrator of the server, that uniquely identifies a user (or group of users), and domain is a DNS name.
The e-mail can contain any character alphabetical and numerical (excluding accents) and some symbols like the underscore (_) and dot (.). Very often can be useful to redirect users to use the services used to automatically forward all messages that arrive on the mailbox, to another of their choice, so that when the consultation must not access all the boxes e-mail available, but a check is sufficient.
System Architecture of e-mail
The basic components of the system of e-mails are:
- Clients (known in jargon MUA, Mail User Agent) used to access a mailbox and send messages
- Servers, which serve two main functions:
- or store messages for one or more users (known in jargon MS Message Store)
- or receive messages and incoming and outgoing channel it (called in the jargon MTA, Mail Transfer Agent).
The protocols typically used for exchanging emails are SMTP, used for sending, receiving and forwarding messages between servers, POP and IMAP, used for receiving messages and consultation by users.
Clients require the configuration of the server to contact, and are therefore suitable mainly used computers regularly. It is also very common to consult a mailbox through the web.
An e-mail consists of:
- An envelope (envelope)
- A section header (header)
- A message body (body)
The benchmark is the RFC, RFC 822, and RFC 2822 (he has made obsolete the previous year).
Env
For envelope is the information supplied with the message that is exchanged between servers via SMTP, mainly e-mail sender and recipients. This information normally corresponds to those that you can find in the headers, but it may be different.
Headers
Headers are information service used to control the sending of the message, or keep track of the manipulations affected. Each header consists of a line of text with a name followed by ‘:’ and the corresponding value.
Some of these are defined by the user. Among the main ones include:
- Subject: (Subject:) should contain a brief description of the message. It is considered good manners to use this field to help the recipient understand the message content.
- From: (From:) contains the e-mail address.
- To: (A) contains the email addresses of the main targets.
- Cc: contains the e-mail addresses in the carbon copy (carbon copy).
- Bcc (Bcc:) contains the e-mail addresses in copy hidden knowledge (blind carbon copy) or recipients who receive the message but whose address does not appear among the recipients. This is actually a pseudo-header, since it is only visible to the sender of the message, and by definition is not shown in messages sent to recipients.
- Reply-to: (Reply:) contains the e-mail address to which must be sent any replies to the message, if different from sender.
- Date: (Date:) contains the date and time the message was written.
Headers of service
Other headers are added by programs that manipulate the message.
The most important is the Received: header which is added to each SMTP server handling the message, specifying which IP address the message was received, what time, and other useful information to trace the route.
Other headers signal that the message was rated by some kind of antivirus or spam filter automatically, and the assessment made by the filter.
Message-ID: (ID of the message) is a code constructed by the client on which the message was composed, which are expected to uniquely identify a message.
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