Some Yahoo! services, like Yahoo! Mail, require you to periodically enter your password even though you are already signed in. We call this Password prompting. This is added security so your information will stay safe.
You can set how often you will be asked by Yahoo! to supply your password for other areas of Yahoo!.
The password prompting option can be changed by visiting your Yahoo! Account Information at Member Information.
Note: You will be asked to sign in anytime you want to edit your account. Choose from the pull-down menu how often you would like to be prompted for your password.
Yahoo! Mail users:
If you are using a desktop shortcut, bookmark, or browser Favorite to access the Yahoo! Mail sign-in page, or if you have Yahoo! Mail set as your home page, please make sure you are specifically going to Yahoo! Mail and not a different URL that contains any additional characters. You might need to reset your bookmark to the above-mentioned URL.
You can also go to the Yahoo! home page and click the "Mail" link. This will take you to the correct sign-in page.