209 lines
4.0 KiB
HTML
209 lines
4.0 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<HTML
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><HEAD
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><TITLE
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>Forwarded Mail</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
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REL="HOME"
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TITLE="Spam Filtering for Mail Exchangers"
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HREF="index.html"><LINK
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REL="UP"
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TITLE="Considerations"
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HREF="considerations.html"><LINK
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REL="PREVIOUS"
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TITLE="Blocking Access to Other SMTP Servers"
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HREF="otherservers.html"><LINK
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REL="NEXT"
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TITLE="User Settings and Data"
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HREF="usersettings.html"></HEAD
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><BODY
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CLASS="section"
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BGCOLOR="#FFFFFF"
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TEXT="#000000"
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LINK="#0000FF"
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VLINK="#840084"
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ALINK="#0000FF"
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><DIV
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CLASS="NAVHEADER"
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><TABLE
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SUMMARY="Header navigation table"
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WIDTH="100%"
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BORDER="0"
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CELLPADDING="0"
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CELLSPACING="0"
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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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>Spam Filtering for Mail Exchangers: </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="otherservers.html"
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ACCESSKEY="P"
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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 3. Considerations</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="usersettings.html"
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ACCESSKEY="N"
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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="section"
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><H1
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CLASS="section"
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><A
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NAME="forwardedmail"
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></A
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>3.3. Forwarded Mail</H1
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><P
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> You should take care not to reject mail as a result of spam
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filtering if it is forwarded from <SPAN
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CLASS="QUOTE"
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>"friendly"</SPAN
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>
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sources, such as:
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</P
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><P
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></P
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><UL
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><LI
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><P
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> Your backup MX hosts, if any. Supposedly, these have
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already filtered out most of the junk (see <A
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HREF="multimx.html"
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>Multiple Incoming Mail Exchangers</A
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>).
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</P
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></LI
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><LI
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><P
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> Mailing lists, to which you or your users subscribe. You may
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still filter such mail (it may not be as criticial if it
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ends up in a black hole). However, if you reject the mail,
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you may end up causing the list server to automatically
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unsubscribe the recipient.
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</P
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></LI
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><LI
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><P
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> Other accounts belonging to the recipient. Again,
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rejections will generate collateral spam, and/or create
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problems for the host that forwards the mail.
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</P
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></LI
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></UL
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><P
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> You may see a logistical issue with the last two of these
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sources: They are specific to each recipient. How to you allow
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each user to specify which hosts they want to whitelist, and
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then use such individual whitelists in a system-wide SMTP-time
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filtering setup? If the message is forwarded to several
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recipients at your site (as may often be true in the case of
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a mailing list), how do you decide whose whitelist to use?
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</P
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><P
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> There is no magic bullet here. This is one of those situations
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where we just have to do a bit of work. You can decide to
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accept all mails, regardless of spam classification, so long as
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it is sent from a host in the whitelist of any one of the
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recipients. For instance, in response to each <B
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CLASS="command"
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>RCPT
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TO:</B
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> command, we can match the sending host against the
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corresponding user's whitelist. If found, set a flag that will
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prevent a subsequent rejection. Effectively, you are using an
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<EM
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>aggregate</EM
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> of each recipient's whitelist.
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</P
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><P
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> The implementation appendices cover this in more detail.
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</P
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></DIV
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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="100%"><TABLE
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SUMMARY="Footer navigation table"
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WIDTH="100%"
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BORDER="0"
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CELLPADDING="0"
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CELLSPACING="0"
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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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><A
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HREF="otherservers.html"
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ACCESSKEY="P"
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>Prev</A
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></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="index.html"
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ACCESSKEY="H"
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>Home</A
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></TD
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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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><A
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HREF="usersettings.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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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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>Blocking Access to Other SMTP Servers</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="considerations.html"
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ACCESSKEY="U"
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>Up</A
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></TD
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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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>User Settings and Data</TD
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></TR
|
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></TABLE
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></DIV
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></BODY
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></HTML
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> |