Yahoo added "ymail" to expand the number of email addresses available to its users.
Please be aware that if you have registered your Lookout account using a "ymail.com" email address you must use that exact address to log into your account subsequently. It is not interchangeable with a "yahoo.com" address. You can, however, change the email address for your account once you are logged in. Please see instructions below.
If you need to change your registered email address, please see here.