Ability to search messages using a "to:" search modifier. It's great to be able to search for messages "from:" a certain user, but it would be great to have "to:" as well.

This would be helpful, for example, to help prevent me from asking team members repetitive questions. I know that a certain team member is the subject matter expert for a given topic. I know I've probably asked this team member a given question before at some point. If I could search "to:username given_topic" that seems the most logical way to find the last time I asked him this question.

To implement a "to:" search feature, it would seem to make the most sense that a "to:" search be confined to either 1) the set of all direct messages or 2) the direct messages with the given user.