It will only work if you "Always" "Accept third-party cookies and site data". Choosing "From visited" will block gmail but still allow Hotmail and Yahoo Mail. No help on the website for this, I figured it out by trial and error. Spent months without X-notifier thus 3 stars.
It doesn't work, it worked well for the first couple of years then stopped notifying me of new emails. If I click on my email provider in the drop down window it takes me to my email account but it doesn't notify me of new emails which is the whole point of this add on !