Being someone who did not have a chance to work with Exchange prior to Exchange 2003, there are aspects of Exchange, even today, that seem to stem from back in time, some that I don't fully understand in terms of how do they play and why. On that list, Legacy Exchange DN and X500 happen to be two items.
Both LegExDN and X500 intrigued me for quite some time now. But not any more I guess!
Just a few minutes ago during a random innocuous search, I stumbled on this blog post by the product team from back in time about LegDN and X500 addresses, finally ending a small part of my larger quest.
It gives the background and explains why LegDN is so important and how X500 works.
My key takeaway was that Outlook internally uses LegDN for mail routing. I kind of understood this before but I definitely have a better understanding now.