Using Outlook Search Folders to filter only external mail

Following on from my recent post about search folders, I got a few questions and comments via mail. One, from Christian, asked if it was possible to use a search folder to filter out only mail which came from “outside” – eg Only show me the last day’s mail from external senders, thus filtering out all the organisational spam that will typically be clogging the mailbox.

I had a bit of a scratch of the old bonce, and figured out one possible way – there may be others, but none are really obvious. There are a couple of requirements for this to work out, though…

  • Your inbound mail must be handled by Exchange 2003 or 2007, using the Intelligent Message Filter technology to stamp inbound mail with a Spam Confidence Level value. It may be possible to use other tools which also support the Exchange Anti-Spam Framework.
  • You need to install a form into Outlook, which will expose that SCL property which is stamped onto inbound mail. This will then allow you to add the SCL value to views, filters etc.

Once you’ve got the SCL form installed, you can create a search folder with the appropriate length filter (eg all mail since yesterday morning, as described on my previous post) and add a new filter to restrict the SCL value (click on the Field drop-down, and look for either the name you gave the SCL form, or the “Forms…” link to add that).

If you set the filter to be at least “0”, that means that any message with an SCL value of 0 or higher will be shown (you might want to add an additional filter to not show stuff with a high SCL value, since they’re lkely to be in your junk mail folder anyway). You could also restrict which folders are shown, based on the same logic in my earlier post.

Anyway, mail with an SCL value will have come from an external source (ie internal Exchange<->Exchange mail won’t have been scanned by the IMF at all), and will have come from an anonymous connection (mail sent by authenticated servers through the same gateway will be assigned an SCL of -1 to show that it’s exempt from filtering by the SmartScreen logic within the IMF).

It might take a little while to render the search folder if you’ve a large mailbox, but it does provide a nice way of showing you only stuff which came from outside.


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