I keep getting a error that says: Messages could not be received at this time.  The server for account "Windstream" returned the error "invalid user name or password.." Your username/password or security settings may be incorrect. Would you like to try re-entering your password?

Sometimes I close everything down and start over and other times I just try sending an email and it works and then after that all is well for a few days.  Anyone know what this is about?  I'm using Office 2011 for Mac.