It is possible that someone without your knowledge is using your email account that's why you've reached the maximum daily sending limit of your account.
To ensure that you’re the only person who can access and manage your account, ensure I suggest that you enhance your account security. Here’s how:
I. I suggest that you change your password as well as your secret question and answer regularly to increase the privacy of your account. To learn how to do this, please visit these links:
http://windowslivehelp.com/solutions/accounts/archive/2008/10/27/how-do-i-reset-my-password.aspx
http://windowslivehelp.com/solutions/password/archive/2008/08/20/password-reset-basics.aspx
http://windowslivehelp.com/solutions/safety/archive/2009/04/29/recent-reports-of-account-hijacks.aspx
II. Properly log out of your account after your Windows Live Hotmail session by clicking the "Sign Out" button on the upper-right side of your Windows Live Hotmail page.
III. If you are using a public terminal, I suggest that you clear your temporary Internet files (cache) and close the browser that you are using when you are ready to end your Internet session.
http://windowslivehelp.com/solutions/accounts/archive/2009/03/03/How-to-optimize-Internet-Explorer-browser.aspx
For other browsers, please refer to the Help files of your browser.
IV. Please do not share your password to anyone even if they claim that they are persons of authority. Windows Live Hotmail disclaims all attempts to gather account information from you via phone or e-mail.
V. Be sure not to click "Remember my password" at the Windows Live Hotmail Sign-in page if you are using a public computer or if there are other people using your computer. Enabling this feature will allow users to automatically sign in to their account and bypass the username and password authentication portion.