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Junk Email Filters


sam13484

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Can someone help me understand how Junk Email Filters work. I have Outlook 2003

installed, but looking through all the drop-down menus and dialog boxes, I

can't find any references to the installed filters. Even when I update them,

it's the same. I thought that there would be somewhere in Outlook to

actually view the filter contents, or at least check which version of the

filter is installed. I can see where I can enter my own filter elements, but

there's not even a clue about the MS filter, where it is installed, what it

blocks, the version number and so on.

Can anyone enlighten me about this. Thanks.

Edited by sam13484
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Can someone help me understand how Junk Email Filters work. I have Outlook 2003

installed, but looking through all the drop-down menus and dialog boxes, I

can't find any references to the installed filters. Even when I update them,

it's the same. I thought that there would be somewhere in Outlook to

actually view the filter contents, or at least check which version of the

filter is installed.

No, the only thing you can "see" in the Outlook user interface are "Blocked Senders" (your personal blacklist), "Safe Senders" (your personal whitelist), and "Rules" (your personal keywords and other custom criteria). The Microsoft filter data is not visible. You can check the version of your filter by looking at 2 files under Program Files\Microsoft Office\OFFICE11:

  • OUTLFLTR.DAT for Outlook 2003 U.S. English is currently version 1.4.5618.83 for the September 2007 release (digitally signed August 22, 2007). This file contains modified MD5 hash values for over 100,000 key words and some corresponding weight criteria for each word (whether it is used in subject or body, what day of week was the message sent on, etc). Microsoft uses an automated process to generate these word lists based primarily on the most common spam messages hitting Hotmail lately (over 60 billion messages per month if you count all languages). In essence, it's a Bayesian filter which is being "trained" monthly by hundreds of millions of people worldwide.
  • OUTLFLTR.DLL for Outlook 2003 U.S. English is currently version 1.4.3504.0 and has not changed since SP1 (digitally signed November 4, 2005).

Since the actual words are not stored in the DAT file, it is not possible to view them in any way. However, since most (not all) of the words are regular English words which can be found in a dictionary, brute force can be used on the hash values to determine the majority of them. Serious spammers undoubtedly go to that trouble, but otherwise the list is long and boring, changes monthly, and cannot be altered without corrupting the digital signature.

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