well your host could have updated or changed your server and turned off php mail. Do you get any emails. Admin emails if you choose to recieve email on all photos and comments?
Basically we use the php mail string. As long as you enter a valid email and turn on notifications and select to be notified we will send an email. Now of course as noted your server has to be setup to send php mail.
You would need to recheck all your settings in admin for notifications etc and test this. If you continue to get no emails and things are set correctly you need to contact your webhost to assist. There is not much we can do to assist with your hosting setup.