How to determine what Outlook rule was applied?

Move the email back into the inbox. Open up the the rules manager and run the rules one by one which is easy to do in the 'Run Rules Now' dialog. You'll see it immediately disappear.

One fairly low impact way might be to change the rule actions to do it's usual thing to the messages (such as move them to a folder) and add a category to them.

Create a category for each rule you might need to debug (or all of them), then for any given email you can quickly see which rule(s) have been applied to it by checking the categories. I would suggest setting the colour of all the categories used for this purpose to the same thing, such as a very neutral pale grey.

I am not sure exactly how much infomation you can pick out of outlook's logging feature, but it might help. I just enabled this on one of my machines and going to see if it shows me my rules in action. To enable the logging:

1. Under the Tools menu, click Options.

2. On the Other tab, click Advanced Options.

3. Check (enable) or uncheck (disable) the Enable logging (troubleshooting) box.

4. Click OK twice and restart Outlook. Logging Enabled should appear in the title bar.

This applies to outlook 2007. The log file is created at C:\Users\user name\AppData\Local\Temp\Outlook Logging\OPMLog.log in Windows Vista. For Windows XP users, it will be created at C:\Documents and Settings\user name\Local Settings\Temp\Outlook Logging\OPMLog.log

You can try disabling all the rules, moving a message that was affected back to the Inbox, then running one of the possibly-problematic rules on the Inbox to see if it does anything to the message. If not, then try again with one of the other rules. Kind of a clunky debug, but it should help to identify while rule is the culprit.