Go to your AdminCP -> Styles & Templates -> Select your style -> Modify User Option Templates -> modifyoptions
Find:
HTML Code:
<input type="checkbox" name="options[emailonpm]" value="1" id="cb_emailonpm" $checked[emailonpm] />
Replace with:
HTML Code:
<input type="checkbox" name="options[emailonpm]" value="1" id="cb_emailonpm" $checked[emailonpm] checked />
Save.