Email on Droid

I have had an HTC Sensation for almost 3 years now with multiple email accounts linked to the device (Gmail, Yahoo, and hotmail).  All have worked flawlessly until last week, where my hotmail account does not allow me to send messages from my phone.  I still receive emails and it updates regularly, but will not send anything.  I have attempted to reconfigure the settings, and sign in again, but it keeps giving me an authentication error message.  Which is odd considering I remain signed in, and receive mail.