email when PM's default
 

How can I make this a default checked automatically for all users?
"Receive Email Notification of New Private Messages"
http://screencast.com/t/ogN2ndGfSo7

I think there's an automatic query for that in Maintenance>Execute SQL Query. Note you must enable this ability (running SQL querys) for yourself (see the manual).

Note also that on busier boards this could lead to serious overload issues. e.g. Many hosts only allow x emails per hour from shared servers.

What Videx suggested will set all the existing registered users to have that option checked. For future users, you should go to vboptions > User registration Options > Default registration options and check that item there.