Are you sharing a computer? Unfortunately, if you don't sign out completely every time you leave the computer, someone can log into your account. Also, they can just hit the back button or the history function to get into it. Change your password and sign out completely to lock them out.
Here are a couple of things you can do to protect your self.
SET UP YOUR SPAM PROTECTION
The location to block spam emails in under the "Options" link.
1. Click on the "Options", Mail Options link in the upper right of your yahoo mail page.
2. Click on "Spam Protection", fill out the form and click the "Save Changes" link.
Once you have this set up, each time you see spam or mail that you want to block, check the box next to it, go to the top of the email list and click the "Spam" button.
Note: Every message you report trains your Spam Guard filters to work better for you. The messages you select in this way will be deleted and reported as spam.
The senders of the messages will be added to your "Blocked Addresses" list.
CREATE A SIGN IN SEAL
Don’t be an easy target of phishing scams. Protect yourself from password theft and fight phishing. It’s free! A sign-in seal is a secret message or image that you create to help protect your Yahoo! Account from Phishing. Phishing - a play on the word “fishing” –is an attempt to steal your password and private account information.
Phishing is when a fraudster sends you an email or link, claiming to be from a legitimate company. They’ll often send you to a fake website they’ve created, and trick you into entering your password and personal information.
Phishers create fake websites that look like those of legitimate companies like Yahoo to trick you into disclosing your user name and password. These pages often look identical to the real thing, so it can be hard to tell that you are on a phony or “spoofed” site.
Creating a Yahoo sign-in seal for your computer will help you ensure that you’re on a legitimate Yahoo sign in page each time you use that computer to sign in to Yahoo. Just look for the custom text or image you set up on the computer. If it’s not there, you might have landed on a “spoofed” site.
Your sign-in seal is saved on the computer you created it on. If you use more than one computer or browser, you should create a sign-in seal for each one. So if you create a Yahoo sign-in seal on your computer at work, we suggest you create one on your home computer, too.
Important: Don’t create a Yahoo sign-in seal on a computer you share with strangers, such as those in libraries, Internet cafes, and other public places.
Creating a sign-in seal is fast and easy. Once you do, look for it every time you sign-in to make sure you’re on a genuine Yahoo site.
If your seal does not appear, beware. You could be on a fraudulent site. Although there may be other reasons you’re not seeing it. Use this link and follow the prompts to create one. https://protect.login.yahoo.com/