439 lines
7.9 KiB
HTML
439 lines
7.9 KiB
HTML
<HTML
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><HEAD
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><TITLE
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>Electronic Mail</TITLE
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>Linux Network Administrators Guide</TH
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WIDTH="10%"
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>Prev</A
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WIDTH="10%"
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><A
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HREF="x-087-2-mail.message-format.html"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="CHAPTER"
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><H1
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><A
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NAME="X-087-2-MAIL"
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>Chapter 17. Electronic Mail</A
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></H1
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><DIV
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CLASS="TOC"
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><DL
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><DT
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><B
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>Table of Contents</B
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></DT
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><DT
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>17.1. <A
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HREF="x-087-2-mail.message-format.html"
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>What Is a Mail Message?</A
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></DT
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><DT
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>17.2. <A
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HREF="x-087-2-mail.delivery.html"
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>How Is Mail Delivered?</A
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></DT
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><DT
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>17.3. <A
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HREF="x-087-2-mail.address.html"
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>Email Addresses</A
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></DT
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><DD
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><DL
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><DT
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>17.3.1. <A
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HREF="x-087-2-mail.address.html#X-087-2-MAIL.ADDRESS.RFC822"
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>RFC-822</A
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></DT
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><DT
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>17.3.2. <A
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HREF="x-087-2-mail.address.html#X-087-2-MAIL.ADDRESS.OBSOLETE"
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>Obsolete Mail Formats</A
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></DT
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><DT
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>17.3.3. <A
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HREF="x-087-2-mail.address.html#X-087-2-MAIL.ADDRESS.MIXING"
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>Mixing Different Mail Formats</A
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></DT
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></DL
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></DD
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><DT
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>17.4. <A
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HREF="x-087-2-mail.routing.html"
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>How Does Mail Routing Work?</A
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></DT
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><DD
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><DL
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><DT
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>17.4.1. <A
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HREF="x-087-2-mail.routing.html#X-087-2-MAIL.ROUTING.INTERNET"
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>Mail Routing on the Internet</A
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></DT
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><DT
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>17.4.2. <A
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HREF="x-087-2-mail.routing.html#X-087-2-MAIL.ROUTING.UUCP"
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>Mail Routing in the UUCP World</A
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></DT
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><DT
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>17.4.3. <A
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HREF="x-087-2-mail.routing.html#AEN14362"
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>Mixing UUCP and RFC-822</A
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></DT
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></DL
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></DD
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><DT
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>17.5. <A
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HREF="x-087-2-mail.elm.html"
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>Configuring elm</A
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></DT
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><DD
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><DL
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><DT
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>17.5.1. <A
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HREF="x-087-2-mail.elm.html#X-087-2-MAIL.ELM.GLOBAL"
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>Global elm Options</A
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></DT
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><DT
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>17.5.2. <A
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HREF="x-087-2-mail.elm.html#X-087-2-MAIL.ELM.CHARSETS"
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>National Character Sets</A
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></DT
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></DL
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></DD
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></DL
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></DIV
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><P
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>Electronic mail transport has been one of the most prominent uses of
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networking since the first networks were devised. Email started as a
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simple service that copied a file from one machine to another and
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appended it to the recipient's <I
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CLASS="EMPHASIS"
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>mailbox</I
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> file. The
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concept remains the same, although an ever-growing net, with its
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complex routing requirements and its ever increasing load of messages,
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has made a more elaborate scheme necessary.</P
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><P
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>
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Various standards of mail exchange have been devised. Sites on the
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Internet adhere to one laid out in RFC-822, augmented by some RFCs
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that describe a machine-independent way of transferring just about
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<I
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CLASS="EMPHASIS"
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>anything</I
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>, including graphics, sound files, and
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special characters sets, by email.<A
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NAME="X-087-2-FNMA01"
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HREF="#FTN.X-087-2-FNMA01"
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>[1]</A
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>
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CCITT has defined another standard, X.400. It is still used in some
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large corporate and government environments, but is progressively being
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retired.</P
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><P
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>
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Quite a number of mail transport programs have been implemented for
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Unix systems. One of the best known is <B
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CLASS="COMMAND"
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>sendmail</B
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>,
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which was developed by Eric Allman at the University of California at
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Berkeley. Eric Allman now offers <B
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CLASS="COMMAND"
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>sendmail</B
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> through a
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commercial venture, but the program remains free
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software. <B
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CLASS="COMMAND"
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>sendmail</B
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> is supplied as the standard mail
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agent in some Linux distributions. We describe <B
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CLASS="COMMAND"
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>sendmail</B
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>
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configuration in <A
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HREF="x-087-2-sendmail.html"
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>Chapter 18</A
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>.</P
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><P
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>
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Linux also uses <B
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CLASS="COMMAND"
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>Exim</B
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>, written by Philip
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Hazel of the University of Cambridge. We describe <B
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CLASS="COMMAND"
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>Exim</B
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>
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configuration in <A
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HREF="x-087-2-exim.html"
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>Chapter 19</A
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>.</P
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><P
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>Compared to <B
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CLASS="COMMAND"
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>sendmail</B
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>, <B
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CLASS="COMMAND"
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>Exim</B
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> is rather
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young. For the vast bulk of sites with email requirements, their capabilities
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are pretty close.</P
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><P
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>Both <B
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CLASS="COMMAND"
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>Exim</B
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> and <B
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CLASS="COMMAND"
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>sendmail</B
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> support a
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set of configuration files that have to be customized for your
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system. Apart from the information that is required to make the mail
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subsystem run (such as the local hostname), there are many parameters
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that may be tuned. <B
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CLASS="COMMAND"
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>sendmail</B
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> 's main
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configuration file is very hard to understand at first. It looks as if
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your cat has taken a nap on your keyboard with the shift key pressed.
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<B
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CLASS="COMMAND"
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>Exim</B
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> configuration files are more structured and
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easier to understand than
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<B
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CLASS="COMMAND"
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>sendmail</B
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> 's. <B
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CLASS="COMMAND"
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>Exim</B
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>,
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however, does not provide direct support for UUCP and handles only
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domain addresses. Today that isn't as big a limitation as it once might
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have been; most sites stay within <B
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CLASS="COMMAND"
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>Exim</B
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>'s
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limitations. However, for most sites, the work required in setting up
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either of them is roughly the same.</P
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><P
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>In this chapter, we deal with what email is and what issues administrators
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have to deal with.
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<A
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HREF="x-087-2-sendmail.html"
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>Chapter 18</A
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> and <A
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HREF="x-087-2-exim.html"
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>Chapter 19</A
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> provide instructions on setting
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up <B
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CLASS="COMMAND"
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>sendmail</B
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> and <B
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CLASS="COMMAND"
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>Exim</B
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> and for the
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first time. The included information should help smaller sites
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become operational, but there are several more options and you can
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spend many happy hours in front of your computer configuring the
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fanciest features.</P
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><P
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>Toward the end of this chapter we briefly cover setting up
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<B
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CLASS="COMMAND"
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>elm</B
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>, a very common mail user agent on many
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Unix-like systems, including Linux.</P
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><P
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>
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For more information about issues specific to electronic mail on Linux, please
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refer to the Electronic Mail HOWTO by
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Guylhem Aznar,<A
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NAME="X-087-2-FNMA02"
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HREF="#FTN.X-087-2-FNMA02"
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>[2]</A
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>
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which is posted to
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<SPAN
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CLASS="SYSTEMITEM"
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>comp.os.linux.answers</SPAN
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> regularly.
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The source distributions of <B
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CLASS="COMMAND"
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>elm</B
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>, <B
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CLASS="COMMAND"
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>Exim</B
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>,
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and <B
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CLASS="COMMAND"
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>sendmail</B
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> also contain extensive documentation
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that should answer most questions on setting them up, and we provide
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references to this documentation in their respective chapters. If you need
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general information on email, a number of RFCs deal with this
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topic. They are listed in the bibliography at the end of the book.</P
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></DIV
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><H3
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CLASS="FOOTNOTES"
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>Notes</H3
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><TABLE
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BORDER="0"
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CLASS="FOOTNOTES"
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WIDTH="100%"
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="5%"
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><A
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NAME="FTN.X-087-2-FNMA01"
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HREF="x-087-2-mail.html#X-087-2-FNMA01"
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>[1]</A
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="95%"
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><P
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>Read RFC-1437 if you don't believe this statement! </P
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></TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="5%"
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><A
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NAME="FTN.X-087-2-FNMA02"
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HREF="x-087-2-mail.html#X-087-2-FNMA02"
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>[2]</A
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="95%"
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><P
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>Guylhem can be reached at
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<SPAN
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CLASS="SYSTEMITEM"
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>guylhem@danmark.linux.eu.org</SPAN
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>.</P
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></TD
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></TR
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></TABLE
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><DIV
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CLASS="NAVFOOTER"
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HREF="x13819.html"
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>Prev</A
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></TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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HREF="index.html"
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>Home</A
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HREF="x-087-2-mail.message-format.html"
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>Next</A
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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>Log Files and Debugging</TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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> </TD
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>What Is a Mail Message?</TD
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