Raymond's recent blog on strange things that happen when you let people choose their own name (part 3) reminded me to check if my favorite old email address, kclemson5 AT exchange.microsoft.com was still working: yep, still there.
As to the history of why I can be reached via that email address, it goes back to the months before we shipped Exchange 2000, when we were heavily dogfooding public folders. I had a public folder with my alias (kclemson) that I used to store various messages and documents that I wanted to share with others in my team.
At one point, something somehow happened in MSIT ops  whereby somehow all of the public folders somehow got email enabled, and there was a collision (because gee golly, there already was a kclemson AT exchange.microsoft.com, aka me) so it couldn't give the public folder that address... and somehow it ended up following a codepath that then stamped a 'kclemson5' on my user object. No kclemson4, 3 or 2. Don't ask me why. Ahh, I love software. :-)
So because there's an (understandable) reticence on IT's part to ever remove an email address from a mailbox, it has persisted on my user object ever since.
Also collecting dust on my user object are some additional addresses that are a nice little traipse through memory lane of former Exchange codenames, the history of mail at Microsoft as well as hint at some of the forced complexity we put in our environment in order to ensure that dogfooding Exchange for Microsoft helps us find and fix enterprise-scale bugs before RTM:
And while we're enjoying the many uses of the "E-Mail Addresses" tab when looking up a user in Outlook, here's a fun bit of Exchange trivia: notice how the "smtp:" prefixes on all of those addresses are all lower case? There's a separate proxy address which has a prefix of "SMTP:" in upper case - but only one. The upper case nature of the prefix is how Exchange knows that that proxy is the default proxy address for the user (e.g. the one to use on outbound mail by default). This isn't purely a handy trick to make the item stand out more in the dialog - the code actually looks for the upper case letters.
I wasn't around when these concepts were first implemented in Exchange, but I assume it's one of those pieces of code that was written 15 years ago and well, there really wasn't a compelling reason for changing it because It Worked Fine. If it ain't broke...
 And this is where my memory is fuzzy on the exact details so forgive the number of "somehow"s.  kclemson5 is alive!  I used that same reference recently to a room full of blank looks.