How do I un-ignore an Outlook email thread, and how does it work?

Okay, first to answer the “How to do it” part of the question: To un-ignore an email thread, find a message that is part of the ignored thread and select it. The Ignore button in the ribbon will go into the pressed state to indicate that this thread is being ignored. Click the Ignore button to uncheck it, thereby un-ignoring the thread.

Note that un-ignoring a thread will not un-delete the messages that were ignored when the rule was in effect. But future messages will not be ignored.

Okay, so how does this work internally? If you open your Rules, you won’t find any rules for deleting the ignored thread. Where are the ignored-thread rules kept?

The rules for ignored threads are kept in a hidden folder in your mailbox named Conversation Action Settings, in the Associated Contents table. You can use the MFCMAPI tool to view them.

This trick is handy not just for snooping around under the hood: It’s also useful if you want to un-ignore a thread for which you don’t have a copy in your Deleted Items folder. You can go into the Associated Contents table and delete the rule for the ignored thread. (Note: I’ve never done this myself, but it’s nice to know I could do this if I had to.)

Bonus chatter: Rules for ignored threads expire after 14 days of inactivity, which is nice to know. It means that your Conversation Action Settings folder is self-cleaning to some degree.