Determining internal vs. external email

Sunday, August 26, 2007

I've been trying to get a good Outlook filter to determine internal from external mail. You'd almost think this is a standard request, but it's actually pretty difficult, especially at a large organization like Intel. After some time trying to figure it out and following some bad advice from Microsoft, I ran across Ray Jezek's Blog : Outlook Rules with Exchange.

The idea is fairly sound, but alas, does not work at Intel, where we have many Exchange servers. When a message moves from one server to another, the message gets Internet headers, so the idea of filtering a message header for the word "Received" doesn't work.

I tried other variations on this theme, though, and finally came up with the solution. My rule now matches any message with a message header containing the text of one of the servers listed in's MX records. Internally, our mail will not use one of these hosts, so that's the only sure-fire way to catch external, and only external email that I've found. I suppose if the sender listed the IP address of the mail host my rule would fail, but that's a risk I'm willing to take. ;-)


Anonymous said...

this is interesting. Would an Outlook rule to check to see if the sender's email address has work?

Emil said...

Unfortunately not. If you're on the same exchange server as the sender, there are no Internet headers, and no on the sender's address (the sender is only listed as "Doe, John"). If you're on a different exchange server, the message gets headers and the filter would work, though. So, only partial success. This is one of the many attempts I made.

Steve Smith said...

Great and Useful Article.

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