[ArtONeil]

I've been using Apple Mail for as long as it has existed and suddenly it won't recognize my password. This morning it recognized it, but this afternoon it didn't. I didn't touch my user name and password; I checked the "Caps Lock" key; suddenly, it just wouldn't recognize my password. Oddly enough, my wife has an account on the same system, AND THE SAME PASSWORD—I set it up myself. And her password is recognized. What gives?

[JKT]

That error message is one of the worst on OS X at the moment as it can be extremely misleading. It could just be a simple matter of the server not being active (e.g. if your eMail provider is performing maintenance on their servers or accounts). That is, it potentially has absolutely nothing to do with your username and password. If it persists then there is a problem, but my bet is that if you try again later or tomorrow it will work fine. It'd also be worth your while trying to find out if there is anything wrong with your account/eMail provider. Does it have a Webmail interface as well? Try that first.