This letter is ancient, actually. I remember seeing it a while back, but I can't be certain of the year since I don't remember when exactly I was in my original Neopets craze. I do assume it's fake due to the mis-step in gender and the random capitalization of P in Neopets.
However, when I first saw it, the story attached to it was that there was a hacker (or two) back in the days where user-generated pages (such as lookups and shops) utilized a blacklist system as opposed to the current whitelist to prevent malicious code. The hacker took advantage of a security flaw that slipped past the blacklist and manage to compromise several dozens of accounts. Using those accounts, he proceeded to wreck the economy as much as he could, all the while including his exploit into their shops and lookups to further spread the mayhem. I also remember there was even a username attached, but I can't for the life of me remember what it was.
Looking back, I remember believing this story completely and totally, but now that I've been on the internet for much longer, it's pretty obviously fake or at the very least, the story I heard was greatly exaggerated. If such an event did actually happen, it probably wasn't anything more than a cookie grabber.
I do remember the change to the whitelist system soon after seeing that letter surface, but it was probably just good and believable timing.