No, not wsa:To (there's only one of those). Outlook To:, as in, "the To: line on your email". Apparently, some folks think relative order on the To: line correlates to relative importance in the discussion.

DennisPi has the details, and suddenly all the ./ readers think they know why Vista slipped...

Anyway, while we're on the topic of email addressing arcana, one thing I wish more people would do is respect the colloquial difference between To: and CC:.

Basically, my native expectation is that people who are sending me mail because they want me to do something will put me on the To: line. People who are just including me as an FYI and don't require me to take explicit action should put me on the CC: line.

I have Outlook rules that filter To: vs CC: into different folders, and I prioritize each folder accordingly.

In other words, the two headers are each associated with a different MEP -- To: is synchronous request/reply, CC: is asynchronous request/optional-reply. Of course the synchronous path is subject to varying latency depending on server load :)