424 lines
9.0 KiB
HTML
424 lines
9.0 KiB
HTML
<HTML
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><HEAD
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><TITLE
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>Email Addresses</TITLE
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.57"><LINK
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TITLE="Linux Network Administrators Guide"
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TITLE="Electronic Mail"
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HREF="x-087-2-mail.html"><LINK
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TITLE="How Is Mail Delivered?"
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TITLE="How Does Mail Routing Work?"
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>Linux Network Administrators Guide</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="x-087-2-mail.delivery.html"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Chapter 17. Electronic Mail</TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="x-087-2-mail.routing.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="SECT1"
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><H1
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CLASS="SECT1"
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><A
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NAME="X-087-2-MAIL.ADDRESS"
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>17.3. Email Addresses</A
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></H1
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><P
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>Email addresses are made up of at least two parts. One part is the
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name of a <I
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CLASS="EMPHASIS"
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>mail domain</I
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> that will ultimately
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translate to either the recipient's host or some host that accepts
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mail on behalf of the recipient. The other part is some form of unique
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user identification that may be the login name of that user, the real
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name of that user in “Firstname.Lastname” format, or an
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arbitrary alias that will be translated into a user or list of
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users. Other mail addressing schemes, like X.400, use a more general
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set of “attributes” that are used to look up the
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recipient's host in an X.500 directory server.</P
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><P
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>How email addresses are interpreted depends greatly on what type of
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network you use. We'll concentrate on how TCP/IP and UUCP networks
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interpret email addresses.</P
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="X-087-2-MAIL.ADDRESS.RFC822"
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>17.3.1. RFC-822</A
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></H2
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><P
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>
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Internet sites adhere to the RFC-822 standard, which requires the
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familiar notation of <SPAN
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CLASS="SYSTEMITEM"
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>user@host.domain</SPAN
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>, for which <SPAN
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CLASS="SYSTEMITEM"
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>host.domain</SPAN
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> is the host's fully
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qualified domain name. The character separating the two is properly
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called a “commercial at” sign, but it helps if you read it
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as “at.” This notation does not specify a route to the
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destination host. Routing of the mail message is left to the
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mechanisms we'll describe shortly.</P
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><P
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>You will see a lot of RFC-822 if you run an Internet connected site. Its
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use extends not only to mail, but has also spilled over into other services,
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such as news. We discuss how RFC-822 is used for news in
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<A
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HREF="x-087-2-news.html"
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>Chapter 20</A
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>.</P
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></DIV
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="X-087-2-MAIL.ADDRESS.OBSOLETE"
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>17.3.2. Obsolete Mail Formats</A
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></H2
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><P
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>
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In the original UUCP environment, the prevalent form was
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<SPAN
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CLASS="SYSTEMITEM"
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>path!host!user</SPAN
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>, for which
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<SPAN
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CLASS="SYSTEMITEM"
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>path</SPAN
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> described a sequence of hosts
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the message had to travel through before reaching the destination
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<SPAN
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CLASS="SYSTEMITEM"
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>host</SPAN
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>. This construct is called
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the <I
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CLASS="EMPHASIS"
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>bang path</I
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> notation, because an exclamation mark
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is colloquially called a “bang.” Today, many UUCP-based networks
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have adopted RFC-822 and understand domain-based addresses.</P
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><P
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>
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Other networks have still different means of addressing. DECnet-based
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networks, for example, use two colons as an address separator,
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yielding an address of <SPAN
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CLASS="SYSTEMITEM"
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>host::user</SPAN
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>.<A
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NAME="X-087-2-FNMA05"
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HREF="#FTN.X-087-2-FNMA05"
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>[1]</A
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>
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The X.400 standard uses an entirely different scheme, describing a
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recipient by a set of attribute-value pairs, like country and organization. </P
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><P
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>Lastly, on FidoNet, each user is identified by a code like <SPAN
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CLASS="SYSTEMITEM"
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>2:320/204.9</SPAN
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>, consisting of four numbers
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denoting zone (2 is for Europe), net (320 being Paris and Banlieue),
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node (the local hub), and point (the individual user's PC). Fidonet
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addresses can be mapped to RFC-822; the above, for example, would be written as <SPAN
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CLASS="SYSTEMITEM"
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>Thomas.Quinot@p9.f204.n320.z2.fidonet.org</SPAN
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>. Now didn't we say domain names were easy to remember?</P
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></DIV
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="X-087-2-MAIL.ADDRESS.MIXING"
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>17.3.3. Mixing Different Mail Formats</A
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></H2
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><P
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>It is inevitable that when you bring together a number of different
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systems and a number of clever people, they will seek ways to
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interconnect the differing systems so they are capable of
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internetworking. Consequently, there are a number of different mail
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gateways that are able to link two different email systems together so
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that mail may be forwarded from one to another. Addressing is the
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critical question when linking two systems. We won't look at the
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gateways themselves in any detail, but let's take a look at some of
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the addressing complications that may arise when gateways of this sort
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are used.</P
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><P
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>
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Consider mixing the UUCP style bang-path notation and RFC-822. These
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two types of addressing don't mix too well. Assume there is an address
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of <SPAN
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CLASS="SYSTEMITEM"
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>domainA!user@domainB</SPAN
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>. It
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is not clear whether the <SPAN
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CLASS="SYSTEMITEM"
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>@</SPAN
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>
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sign takes precedence over the path, or vice versa: do we have to send
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the message to <SPAN
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CLASS="SYSTEMITEM"
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>domainB</SPAN
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>,
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which mails it to <SPAN
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CLASS="SYSTEMITEM"
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>domainA!user</SPAN
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>, or should it be sent to
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<SPAN
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CLASS="SYSTEMITEM"
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>domainA</SPAN
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>, which forwards it
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to <SPAN
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CLASS="SYSTEMITEM"
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>user@domainB</SPAN
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> ?</P
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><P
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>
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Addresses that mix different types of address operators are called
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<I
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CLASS="EMPHASIS"
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>hybrid addresses</I
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>. The most common type, which we
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just illustrated, is usually resolved by giving the <SPAN
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CLASS="SYSTEMITEM"
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>@</SPAN
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> sign precedence over the path. In
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<SPAN
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CLASS="SYSTEMITEM"
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>domainA!user@domainB</SPAN
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>, this
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means sending the message to <SPAN
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CLASS="SYSTEMITEM"
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>domainB</SPAN
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> first.</P
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><P
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> However, there is a way to specify routes in RFC-822 conformant ways:
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<SPAN
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CLASS="SYSTEMITEM"
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><@domainA,@domainB:user@domainC ></SPAN
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> denotes the address of <SPAN
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CLASS="SYSTEMITEM"
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>user</SPAN
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> on
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<SPAN
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CLASS="SYSTEMITEM"
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>domainC</SPAN
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>, where
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<SPAN
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CLASS="SYSTEMITEM"
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>domainC</SPAN
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> is to be reached through
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<SPAN
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CLASS="SYSTEMITEM"
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>domainA</SPAN
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> and
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<SPAN
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CLASS="SYSTEMITEM"
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>domainB</SPAN
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> (in that order). This type
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of address is frequently called a <I
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CLASS="EMPHASIS"
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>source routed</I
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> address.
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It's not a good idea to rely on this behavior, as revisions to the
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RFCs describing mail routing recommend that source routing in a mail
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address be ignored and instead an attempt should be made to deliver directly
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to the remote destination.</P
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><P
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>
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Then there is the <SPAN
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CLASS="SYSTEMITEM"
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>%</SPAN
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> address
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operator: <SPAN
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CLASS="SYSTEMITEM"
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>user %domainB@domainA</SPAN
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> is first sent
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to <SPAN
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CLASS="SYSTEMITEM"
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>domainA</SPAN
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>, which expands
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the rightmost (in this case, the only) percent sign to an <SPAN
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CLASS="SYSTEMITEM"
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>@</SPAN
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> sign. The address is now <SPAN
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CLASS="SYSTEMITEM"
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>user@domainB</SPAN
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>, and the mailer happily
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forwards your message to <SPAN
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CLASS="SYSTEMITEM"
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>domainB</SPAN
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>, which delivers it to
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<SPAN
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CLASS="SYSTEMITEM"
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>user</SPAN
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>. This type of address
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is sometimes referred to as “Ye Olde ARPAnet Kludge,” and
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its use is discouraged.</P
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><P
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>There are some implications to using these different types of
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addressing that will be described throughout the following
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sections. In an RFC-822 environment, you should avoid using anything
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other than absolute addresses, such as <SPAN
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CLASS="SYSTEMITEM"
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>user@host.domain</SPAN
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>.</P
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></DIV
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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-FNMA05"
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HREF="x-087-2-mail.address.html#X-087-2-FNMA05"
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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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> When trying to reach a DECnet address from
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an RFC-822 environment, you can use <SPAN
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CLASS="SYSTEMITEM"
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>“host::user"@relay</SPAN
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>, for which
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<SPAN
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CLASS="SYSTEMITEM"
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>relay</SPAN
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> is the name of a known
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Internet-DECnet relay.</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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><HR
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ALIGN="LEFT"
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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><A
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HREF="x-087-2-mail.delivery.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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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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><A
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HREF="x-087-2-mail.routing.html"
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>Next</A
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></TD
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></TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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>How Is Mail Delivered?</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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><A
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HREF="x-087-2-mail.html"
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>Up</A
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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>How Does Mail Routing Work?</TD
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></TR
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></TABLE
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></DIV
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