There's a lot to like about the newly released OS X Mavericks, but like any new OS there are a few bugs. We've noticed that some users are experiencing a problem with the Mail app not remembering their iCloud password when the program is opened. It's annoying to have to input your password every time you fire up the app, so here's a simple work-around.
Open the Mail app. Go to Preferences. Click on the Accounts tab, and then the Advanced tab.
Look for the Authentication box. If you're having this problem, it will be set to "Password." Click on the box and select "Apple Token."
Apple uses Tokens as a secure way to access iCloud services without having to use your password, by providing your username and password to the app over an encrypted SSL connection.