I am creating a social networking site using django, should I have create a email app
You should create separate module. There is no reason to have same function implemented in multiple places, it makes your code hard to maintenance.
Each functionality should have own module (in this case app), so modifying email you will have to modify only one file/module, not all email functions in all modules
I am creating a social networking site using django. I will send lots of emails depending on the user actions. Should I create an email app or just create an email_funcitons.py file in each individual app. Which is best practice? 
Apps created so far:
Accounts
NewsFeed
Profile
Notifications
Messaging
Privacy