wiki:es:Documentation/What is e-mail

Version 1 (modified by natman, 6 months ago) (diff)

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En otros idiomas:


Básico – Qué es correo electrónico?

El correo electrónico fue el primer servicio implementado en Internet y sigue siendo el método más popular de comunicación a través de Internet. Un porcentaje sustancial de personas en Internet sólo utiliza este servicio. Se precide que en los próximos años el correo electrónico sustituirá en muchos propósitos a las formas tradicionales de comuniación como las cartas y el fax. Actualmente uno puede llegar a contactar en pocos minutos con millones de personas en todo el mundo por correo electrónico. Usuarios de otras redes, como Compuserve y T-Online, están conectados a Internet a través de "puertas de enlace".

Composición de una cabecera de mensaje

Cada correo electrónico comienza con una cabecera que está separada del cuerpo del mensaje mediante una línea en blanco. YAM construye esta cabecera automáticamente siguiendo las preferencias especificadas en la configuración.

La cabecera de un correo electrónico se divide en varios campos que comienzan con una línea nueva y tiene generalmente el formato 'Field: contents'. Los campos que son demasiado largos para caber en una línea se pueden dividir en varias líneas. La mayoría de las líneas de cabecera se pueden omitir can be omitted pero se añaden para proporcionar al programa de correo del destinatario información adicional sobre el mensaje o para dar los datos necesarios para comprobar los errores causados ​​por problemas de transmisión. En el siguiente ejemplo se explican unos cuantos campos importantes.

  Return-Path: <just@zfn.uni-bremen.de>

Este campo lo añade el servidor de correo del destinatario y contiene la dirección de correo electrónico del remitente para permitir al ordenador del destinatario enviar una respuesta mediante el correo electrónico.

  Received: from ina.zfn.uni-bremen.de by atlantica.access.ch
            (8.8.5/INA-1.05pri) id XAA29100;
            Tue, 23 Dec 1997 23:40:45 +0100 (MET)
  Received: from moritz37.zfn.uni-bremen.de by
            ina.zfn.uni-bremen.de (AIX 3.2/UCB 5.64/ZFNserver) id AA26355;
            Tue, 23 Dec 1997 23:40:13 +0100

Cada ordenador que envía el mensaje en la siguiente etapa de su recorrido, y también el propio sistema del destinatario, añade un campo "Received:" a la cabecera para indicar cuando ha llegado el mensaje y de donde procede. Esto permite reconstruir problemas de transmisión que puedieran haber afectado al mensaje a lo largo de toda su ruta.

  From: Christian Just <just@example.net>

Dirección de correo electrónico y nombre del remitente.

  Reply-To: just@example.org

La dirección a la que se debe enviar cualquier respuesta a este mensaje. Esto se utiliza si el mensaje es enviado desde un ordenador que el remitente no puede utilizar para leer correo, por cualquier razón. Luego se puede utilizar este campo para indicar dónde se debe enviar una respuesta. En la ausencia del campo Reply-To:, las respuestas van a la dirección indica en el campo From:.

  To: "Marcel Beck (Yet another Mailer-author)" <mbeck@example.com>

La dirección del destinatario; se pueden dar varias direcciones, separadas por comas. Este campo puede contener simplemente la dirección en el formato nombre@dominio o puede ir precedido de un comentario adicional, como el nombre del destinatario -- en este caso, la dirección de correo tiene que ir encerrada entre símbolos (<>).

  Date: Tue, 23 Dec 1997 23:28:30 +0200

La fecha y hora de salida del mensaje. El valor después de la hora representa la zona horaria, expresada como la diferencia respecto a la hora del Meridiano de Greenwich. Los primeros dos dígitos muestran las horas, y los dos siguientes los minutos.

  Message-ID: <YAM7296.1549.122414920@zfn.uni-bremen.de>

Identificador único, creado automáticamente en el envío. Usando esto es posible inequívocamente cancelar un mensaje en particular.

  In-Reply-To: <YAM7296.263.121102896@mail.access.ch>

Una referencia precisa para el mensaje que se respondió, como es el Message-ID

  X-Mailer: YAM 2.0beta4 - Amiga Mailer by Marcel Beck - http://yam.ch

Nombre y Versión del programa de correo del remitente.

  Subject: Re: YAM2beta5

Este campo debe contener concisamente el tópico del mensaje. Da un asunto claro e informativo como marca de buen estilo Netiquette.

 Mime-Version: 1.0
 Content-Transfer-Encoding: quoted-printable
 Content-Type: text/plain; charset=iso-8859-1

Estos campos indican que el mensaje se envía en formato MIME. En este ejemplo, contiene un texto en juego de caracteres ISO-Latin-1 y caracteres que no pueden ser representados usando 7 bits son codificados como 'Quoted-printable'.

Protocolos de Correo

YAM utiliza el "Simple Mail Transfer Protocol" (SMTP) para enviar su correo al servidor SMTP, el cual también utiliza SMTP para transmitir su correo a través del mundo. Los correos entrantes llegan a su servidor "Post Office Protocol" (POP), donde espera hasta que YAM los descarga en su ordenador, utilizando el protocolo POP3 (POP versión 3). Los mensajes que YAM envía y recibe conforme a los convencionalismos fijados en RFC 822 y RFC 1521 (MIME).

Correo Saliente

If you want to send an e-mail message to anyone, YAM transfers the message by SMTP to your local SMTP server. This computer forwards the message to the recipient's computer, generally also by SMTP.

Why does YAM not deal directly with the recipient's server ? Firstly, it would take quite a long time for your Amiga to get a connection to one particular computer and then transmit the message. Secondly, many computers are hard to find; it is much better to let the mail server look for the address, instead of burdening your Amiga. Thirdly, quite frequently the recipient's server will not be available at the time you want to send the mail. The SMTP server solves these problems, holding back the message until the other computer is ready to receive it.

Correo Entrante

If someone sends you email, the other computer transfers it using the SMTP protocol as far as your POP server. This stores the message in a sort of mailbox, where it remains until YAM collects it. When you look for new mail, YAM downloads the message to your Amiga using POP3.

Why doesn't YAM use SMTP for incoming mail? SMTP works best if both computers are ready to receive messages. Unless you run YAM and your Amiga 24 hours a day and seven days a week, SMTP would not be particularly suitable for you.

Qué es MIME?

MIME stands for Multipurpose Internet Mail Extensions. MIME serves two main purposes: it allows one mail application to tell another what sort of data is contained in a message, and it also provides standardised rules by which mail applications can encode data, so that it can be sent through the Internet mail system.

Codificación MIME

The Internet uses the SMTP protocol to move mail around. SMTP is limited to the US-ASCII character set. This is a problem for people who speak languages other than American English and so need accented characters or non-American letters, or for people who want to use special symbols like the bullet. Even more difficult is the transmission of binary files, as it is often the case with attachments. MIME provides a way around this restriction by offering two encodings: quoted-printable and base64.

These encodings use US-ASCII character codes to represent any sort of data you like, including special characters or even non-text data. Quoted-printable is used for data that is mostly text, but has special characters for very long lines.

Quoted-printable looks just like regular text, except when a special character is used -- the special character is replaced with a "=" (dash) and two more characters that represent the (hexadecimal) character code of the special character. Thus, a bullet in quoted-printable looks like =95DA.

No line in quoted-printable is allowed to be more than 76 characters long. If your mail has some line longer than 76 characters, the quoted-printable encoding will break your line in two, and put a "=" at the end of the first line, to notify the mail reader at the other end that the two lines are really supposed to be one.

Base64 encoding is another way to protect binary data from the SMTP mail system. However, Base64 makes no attempt to be readable, and is more appropriate for non-text data. It is equivalent to the older UUencode, but more reliable in use.

Tipo de Contenido

The other important function of MIME is to allow mail programs to exchange information about what kind of data is in a message (or part of a message). The primary mechanism used for this is the Content-Type: header. The main content types are:

  text        readable text
  image       pictures and graphics
  audio       sound
  video       animations
  message     messages or parts of messages
  multipart   several different kinds of data in a single message

The subtype gives additional information about the type of data:

  text/plain   plain text
  text/html    text in HTML format
  image/gif    image in GIF format
  etc.

By looking at the Content-Type: header, a mail program can select the most suitable utility to display an attached file.

Encriptado con PGP

In order to ensure that the e-mail cannot be read by anyone other than the recipient, it is necessary to encrypt the transmission. Is this important? Sometimes very much so! It is not possible to say in advance what route electronic mail will take through the Net and along the way it is possible for someone to read your mail unauthorised, admittedly with more trouble than one would normally bother to take. In particular, encrypting e-mail is a wise precaution if you want to send passwords, credit card numbers or some such over the Net. Such encrypted data is then often transmitted more safely than if sent by normal letter post. A simple, effective and widely used tool for this sort of encryption is PGP, short for Pretty Good Privacy.

PGP was developed by Phil Zimmermann and employs the public key method. Using this PGP program, one can be sure that the message is the one actually written by the sender, and that only the intended recipient can read it. The so called public keys offer the highest possible level of security.

There are two kinds of key:

  • One is a private key, used on your computer and never revealed elsewhere.
  • The other sort is the public key. You can make as many copies of this as you like, and send the copies to other users so that they can send you encrypted mail.

You need both types of key, public and private, because they are inherently connected together. You can distribute your public key as often as needed, but it will only work when matched up with its exact counterpart. Hence, both public and private keys are involved in locking and (generally) unlocking information.

PGP keys are used in two distinct ways:

  1. Another person can encrypt information using your public key and send the encrypted file to you, to decipher with your private key.
  1. You can encrypt information with your private key and send it safely over the Net. Anyone in possession of your public key can read your communication. The recipient can be sure that the communication is genuinely from you (your digital signature proves its authenticity) and that it has not been altered.

PGP is obtainable as freeware and the International PGP homepage is easy to find on the Internet at http://www.pgpi.org/

Related topics:

  • Installing PGP 2.6.x
  • Installing PGP 5.0i

E-mail netiquette

  • Keep your messages short and to the point.
  • Give the message a concise and meaningful Subject: header, so that it can easily be found again.
  • Put a signature at the end of the message. This should contain your name and e-mail address and should not be longer than five lines. Signatures often also give the postal address, telephone number, website and instructions about sending PGP messages.
  • Only write in block capitals if you want to give particular emphasis to a point. *Stars* are also used for emphasis (YAM interprets this by using bold type). Remember, block capitals are generally taken as equivalent to SHOUTING.
  • Set up YAM to use a line length of less than 80 characters and don't use any control characters.
  • Don't use non-ASCII characters unless you are sure that the recipient's software interprets them correctly.
  • It is regarded as extremely impolite to forward a private message to a mailing list without the permission of the original sender.
  • Abbreviations and TLAs (Three Letter Acronyms) can be useful provided that they don't make the message unintelligible. The following are often used:

IMHO (in my humble opinion) BTW (by the way) FYI (for your information) AFAIK (as far as I know) ASAP (as soon as possible) CU (see you)

  • Use the ;-) smiley (winky?) to denote a touch of irony which could otherwise easily be misinterpreted.
  • Be tolerant of people's failings, e.g. in spelling, grammar, expression or familiarity with e-mail.
  • If you join a mailing list, read the messages for a while to get a feel of the general style, what questions are asked and what is not suitable.
  • Study the FAQs (lists of Frequently Asked Questions). It is annoying for the members of a group if the same questions are being asked repeatedly.
  • If someone posts a message which is "off-topic", (i.e. it does not belong in the mailing list), reply privately and NOT in the list.
  • If you quote another message, then cut all the bits which are not relevant to your answer. No-one wants to read the same message three or four times, especially when all that is added is "Yes, me too".
  • Resist the temptation to flame in the mailing list. ("Flame" = write abusively, generally when provoked by idiocy.) Remember that the list is public and meant for constructive discussion. Do as you would be done by!
  • If you are replying to a message coming from a mailing list, think carefully whether to send it direct to the sender or to the list, and check that the To: address is correct. It can be very irritating when a letter addressed to a particular person appears on the list, unintentionally.
  • When replying to a message coming from a mailing list, it is usually a good idea to mention the person who wrote that message as part of your welcome phrase, to avoid confusion.