Changes between Initial Version and Version 1 of Documentation/What is e-mail


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Aug 15, 2012 10:26:32 PM (2 years ago)
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damato
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  • Documentation/What is e-mail

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     1= Basics – what is e-mail? 
     2 
     3E-mail was the first service to be implemented on the Internet and 
     4is still the most popular method of communication over the Internet. 
     5A substantial proportion of the people on the Internet use only this 
     6service.  It is predicted that in the next few years e-mail will replace 
     7the traditional forms of communication like letters and fax for many 
     8purposes.  At the present time one can in a matter of minutes reach 
     9several million people world-wide by e-mail.  Users of other networks, 
     10such as Compuserve and T-Online, are connected to the Internet through 
     11"gateways". 
     12 
     13== Make up of a message header 
     14 
     15Every e-mail starts with a header which is separated from the 
     16actual message body by a blank line.  YAM constructs this header 
     17automatically following the settings specified in the configuration. 
     18 
     19The header of an e-mail is divided into fields which each start on 
     20a new line and have the general form 'Field: contents'.  Fields which 
     21are too long to fit in a single line can be split to run over several 
     22lines.  The majority of the header lines can be omitted but are added to 
     23provide the recipient's mail program with additional information about 
     24the message or to give data needed to check for errors caused by 
     25transmission problems.  A few of the important fields are explained by 
     26means of the following example. 
     27 
     28{{{ 
     29  Return-Path: <just@zfn.uni-bremen.de> 
     30}}} 
     31 
     32This field is added by the recipient's mail server and contains 
     33the e-mail address of the sender to allow the recipient's computer to 
     34send a reply by e-mail. 
     35 
     36{{{ 
     37  Received: from ina.zfn.uni-bremen.de by atlantica.access.ch 
     38            (8.8.5/INA-1.05pri) id XAA29100; 
     39            Tue, 23 Dec 1997 23:40:45 +0100 (MET) 
     40  Received: from moritz37.zfn.uni-bremen.de by 
     41            ina.zfn.uni-bremen.de (AIX 3.2/UCB 5.64/ZFNserver) id AA26355; 
     42            Tue, 23 Dec 1997 23:40:13 +0100 
     43}}} 
     44 
     45Each computer which sends the message on the next stage of its 
     46journey, and also the recipient's own system, adds on a "Received:" 
     47field to the header to say when the message arrived and where it 
     48came from.  This allows one to reconstruct transmission problems 
     49which may have affected the message along its route. 
     50 
     51{{{ 
     52  From: Christian Just <just@example.net> 
     53}}} 
     54 
     55E-mail address and real name of the sender. 
     56 
     57{{{ 
     58  Reply-To: just@example.org 
     59}}} 
     60 
     61The address to which any reply to this message should be sent. 
     62This is used if the message is despatched from a computer which the 
     63sender cannot use to read mail, for whatever reason.  Then he can use 
     64this field to say where a reply should be sent.  In the absence of a 
     65'''Reply-To:''' field, replies go to the address given in the '''From:''' field. 
     66 
     67{{{ 
     68  To: "Marcel Beck (Yet another Mailer-author)" <mbeck@example.com> 
     69}}} 
     70 
     71The address of the recipient; additional names can be given, 
     72separated by commas.  This field may contain simply the address 
     73in the form name@domain or else it may be prefaced by an additional 
     74comment, such as the recipient's name -- in this case, the address 
     75must be enclosed in pointed brackets. 
     76 
     77{{{ 
     78  Date: Tue, 23 Dec 1997 23:28:30 +0200 
     79}}} 
     80 
     81Date and time of despatch of the message.  The figure after the time 
     82represents the timezone, expressed as the difference from Greenwich Mean 
     83Time.  Here the first two digits show the hours, and the next two the 
     84minutes. 
     85 
     86{{{ 
     87  Message-ID: <YAM7296.1549.122414920@zfn.uni-bremen.de> 
     88}}} 
     89 
     90An unique identifier, created automatically on despatch.  Using 
     91this it is possible unequivocally to cancel any particular message. 
     92 
     93{{{ 
     94  In-Reply-To: <YAM7296.263.121102896@mail.access.ch> 
     95}}} 
     96 
     97A precise reference to the message being replied, such as its 
     98Message-ID 
     99 
     100{{{ 
     101  X-Mailer: YAM 2.0beta4 - Amiga Mailer by Marcel Beck - http://yam.ch 
     102}}} 
     103 
     104Name and Version of the sender's mail program. 
     105 
     106{{{ 
     107  Subject: Re: YAM2beta5 
     108}}} 
     109 
     110This field should state concisely the topic of the message.  Giving 
     111a clear and informative Subject is a mark of good style Netiquette. 
     112 
     113{{{ 
     114 Mime-Version: 1.0 
     115 Content-Transfer-Encoding: quoted-printable 
     116 Content-Type: text/plain; charset=iso-8859-1 
     117}}} 
     118 
     119These fields indicate that the message is set out in MIME format. 
     120In this example, it contains plain text in the ISO-Latin-1 character 
     121set and characters which cannot be represented using 7 bits are encoded 
     122as 'Quoted-printable'. 
     123 
     124== Mail protocols 
     125 
     126YAM uses the Simple Mail Transfer Protocol (SMTP) in order to send 
     127your mail to the SMTP server, which also uses SMTP to transmit your 
     128mail across the whole world.  Incoming mail arrives at your Post Office 
     129Protocol (POP) server, where it waits until YAM downloads it to your 
     130computer, using the POP3 (POP version 3) protocols.  The messages which 
     131YAM sends and receives conform to the conventions set out in RFC 822 
     132and RFC 1521 (MIME). 
     133 
     134=== Outgoing mail 
     135 
     136If you want to send an e-mail message to anyone, YAM transfers the 
     137message by SMTP to your local SMTP server.  This computer forwards the 
     138message to the recipient's computer, generally also by SMTP. 
     139 
     140Why does YAM not deal directly with the recipient's server ? 
     141Firstly, it would take quite a long time for your Amiga to get a 
     142connection to one particular computer and then transmit the message. 
     143Secondly, many computers are hard to find; it is much better to let the 
     144mail server look for the address, instead of burdening your Amiga. 
     145Thirdly, quite frequently the recipient's server will not be available 
     146at the time you want to send the mail.  The SMTP server solves these 
     147problems, holding back the message until the other computer is ready to 
     148receive it. 
     149 
     150=== Incoming mail 
     151 
     152If someone sends you email, the other computer transfers it using 
     153the SMTP protocol as far as your POP server.  This stores the message 
     154in a sort of mailbox, where it remains until YAM collects it.  When you 
     155look for new mail, YAM downloads the message to your Amiga using POP3. 
     156 
     157Why doesn't YAM use SMTP for incoming mail?  SMTP works best if 
     158both computers are ready to receive messages.  Unless you run YAM and 
     159your Amiga 24 hours a day and seven days a week, SMTP would not be 
     160particularly suitable for you. 
     161 
     162== What is MIME? 
     163 
     164MIME stands for ''Multipurpose Internet Mail Extensions''.  MIME serves 
     165two main purposes: it allows one mail application to tell another what 
     166sort of data is contained in a message, and it also provides 
     167standardised rules by which mail applications can encode data, so that 
     168it can be sent through the Internet mail system. 
     169 
     170=== MIME Encoding 
     171 
     172The Internet uses the SMTP protocol to move mail around.  SMTP is 
     173limited to the US-ASCII character set.  This is a problem for people 
     174who speak languages other than American English and so need accented 
     175characters or non-American letters, or for people who want to use 
     176special symbols like the bullet.  Even more difficult is the 
     177transmission of binary files, as it is often the case with attachments. 
     178MIME provides a way around this restriction by offering two 
     179encodings: '''quoted-printable''' and '''base64'''. 
     180 
     181These encodings use US-ASCII character codes to represent any sort 
     182of data you like, including special characters or even non-text data. 
     183Quoted-printable is used for data that is mostly text, but has special 
     184characters for very long lines. 
     185 
     186Quoted-printable looks just like regular text, except when a 
     187special character is used -- the special character is replaced with 
     188a "'''='''" (dash) and two more characters that represent the (hexadecimal) 
     189character code of the special character.  Thus, a bullet in 
     190quoted-printable looks like '''=95DA'''. 
     191 
     192No line in quoted-printable is allowed to be more than 76 characters 
     193long.  If your mail has some line longer than 76 characters, the 
     194quoted-printable encoding will break your line in two, and put a "'''='''" 
     195at the end of the first line, to notify the mail reader at the other 
     196end that the two lines are really supposed to be one. 
     197 
     198'''Base64''' encoding is another way to protect binary data from the 
     199SMTP mail system.  However, Base64 makes no attempt to be readable, 
     200and is more appropriate for non-text data.  It is equivalent to the 
     201older UUencode, but more reliable in use. 
     202 
     203=== Content type 
     204 
     205The other important function of MIME is to allow mail programs to 
     206exchange information about what kind of data is in a message (or part 
     207of a message).  The primary mechanism used for this is the '''Content-Type:''' 
     208header.  The main content types are: 
     209 
     210{{{ 
     211  text        readable text 
     212  image       pictures and graphics 
     213  audio       sound 
     214  video       animations 
     215  message     messages or parts of messages 
     216  multipart   several different kinds of data in a single message 
     217}}} 
     218 
     219The subtype gives additional information about the type of data: 
     220 
     221{{{ 
     222  text/plain   plain text 
     223  text/html    text in HTML format 
     224  image/gif    image in GIF format 
     225  etc. 
     226}}} 
     227 
     228By looking at the '''Content-Type:''' header, a mail program can select 
     229the most suitable utility to display an attached file. 
     230 
     231== Encrypting with PGP 
     232 
     233In order to ensure that the e-mail cannot be read by anyone other 
     234than the recipient, it is necessary to encrypt the transmission.  Is 
     235this important?  Sometimes very much so!  It is not possible to say in 
     236advance what route electronic mail will take through the Net and along 
     237the way it is possible for someone to read your mail unauthorised, 
     238admittedly with more trouble than one would normally bother to take. 
     239In particular, encrypting e-mail is a wise precaution if you want to 
     240send passwords, credit card numbers or some such over the Net.  Such 
     241encrypted data is then often transmitted more safely than if sent by 
     242normal letter post.  A simple, effective and widely used tool for this 
     243sort of encryption is '''PGP''', short for '''Pretty Good Privacy'''. 
     244 
     245PGP was developed by Phil Zimmermann and employs the ''public key'' 
     246method.  Using this PGP program, one can be sure that the message is 
     247the one actually written by the sender, and that only the intended 
     248recipient can read it.  The so called ''public keys'' offer the highest 
     249possible level of security. 
     250 
     251There are two kinds of key: 
     252 
     253  - One is a private key, used on your computer and never revealed 
     254    elsewhere. 
     255 
     256  - The other sort is the public key.  You can make as many copies of 
     257    this as you like, and send the copies to other users so that they 
     258    can send you encrypted mail. 
     259 
     260You need both types of key, public and private, because they are inherently 
     261connected together.  You can distribute your public key as 
     262often as needed, but it will only work when matched up with its exact 
     263counterpart.  Hence, both public and private keys are involved in 
     264locking and (generally) unlocking information. 
     265 
     266PGP keys are used in two distinct ways: 
     267 
     268   1. Another person can encrypt information using your public key and 
     269      send the encrypted file to you, to decipher with your private key. 
     270 
     271   2. You can encrypt information with your private key and send it 
     272      safely over the Net.  Anyone in possession of your public key 
     273      can read your communication.  The recipient can be sure that 
     274      the communication is genuinely from you (your digital signature 
     275      proves its authenticity) and that it has not been altered. 
     276 
     277PGP is obtainable as freeware and the International PGP homepage 
     278is easy to find on the Internet at http://www.pgpi.org/ 
     279 
     280__Related topics:__ 
     281 
     282 * Installing PGP 2.6.x 
     283 * Installing PGP 5.0i 
     284 
     285== E-mail netiquette 
     286 
     287- Keep your messages short and to the point. 
     288 
     289- Give the message a concise and meaningful '''Subject:''' header, 
     290  so that it can easily be found again. 
     291 
     292- Put a signature at the end of the message.  This should contain 
     293  your name and e-mail address and should not be longer than five 
     294  lines.  Signatures often also give the postal address, telephone 
     295  number, website and instructions about sending PGP messages. 
     296 
     297- Only write in block capitals if you want to give particular 
     298  emphasis to a point.  *Stars* are also used for emphasis (YAM 
     299  interprets this by using bold type).  Remember, block capitals 
     300  are generally taken as equivalent to SHOUTING. 
     301 
     302- Set up YAM to use a line length of less than 80 characters and 
     303  don't use any control characters. 
     304 
     305- Don't use non-ASCII characters unless you are sure that the 
     306  recipient's software interprets them correctly. 
     307 
     308- It is regarded as extremely impolite to forward a private message 
     309  to a mailing list without the permission of the original sender. 
     310 
     311- Abbreviations and ''TLAs'' (Three Letter Acronyms) can be useful 
     312  provided that they don't make the message unintelligible. 
     313  The following are often used: 
     314 
     315            IMHO   (in my humble opinion) 
     316            BTW    (by the way) 
     317            FYI    (for your information) 
     318            AFAIK  (as far as I know) 
     319            ASAP   (as soon as possible) 
     320            CU     (see you) 
     321 
     322- Use the ;-) smiley (winky?) to denote a touch of irony which could 
     323  otherwise easily be misinterpreted. 
     324 
     325- Be tolerant of people's failings, e.g. in spelling, grammar, expression 
     326  or familiarity with e-mail. 
     327 
     328- If you join a mailing list, read the messages for a while to get a 
     329  feel of the general style, what questions are asked and what is 
     330  not suitable. 
     331 
     332- Study the ''FAQs'' (lists of Frequently Asked Questions).  It is 
     333  annoying for the members of a group if the same questions are 
     334  being asked repeatedly. 
     335 
     336- If someone posts a message which is "off-topic", (i.e. it does not 
     337  belong in the mailing list), reply privately and NOT in the list. 
     338 
     339- If you quote another message, then cut all the bits which are not 
     340  relevant to your answer.  No-one wants to read the same message 
     341  three or four times, especially when all that is added is "Yes, me 
     342  too". 
     343 
     344- Resist the temptation to flame in the mailing list.  ("Flame" = 
     345  write abusively, generally when provoked by idiocy.)  Remember that 
     346  the list is public and meant for constructive discussion.  Do as 
     347  you would be done by! 
     348 
     349- If you are replying to a message coming from a mailing list, think 
     350  carefully whether to send it direct to the sender or to the list, 
     351  and check that the ''To:'' address is correct.  It can be very 
     352  irritating when a letter addressed to a particular person appears 
     353  on the list, unintentionally. 
     354 
     355- When replying to a message coming from a mailing list, it is usually 
     356  a good idea to mention the person who wrote that message as part of 
     357  your welcome phrase, to avoid confusion.