How to implement unsubscribe usecase for website
Each new class implies a new query, which adds to the total cost.  Pack as much information that is practical into the User class.  A simple boolean in the User class should work for active/inactive or subscribe/unsubscribe.  Your app needs to accept emails to receive the Unsubscribe request and set the associated boolean to False.
I'm sending automated emails and hence I should deliver an unsubscribe function. I have a User entity that is not used much, only when a user registers and the emails can be send to users who are not registered as Users. So when I send an email and I must include an unsubscribe link, should I keep a whole separate entity / class for class Unsubscriptions or include them as a variable in the User class whether or not a user is registered to receive emails?
Did you use any method for unsubscribe that you can recommend? Are there any frameworks for unsubriptions? GAE that I'm using has a very primitive framework for sending and receiving emails and I understand that Amazon has a much more developed API for manging large email list, but I suppose I can still do it all in GAE without Amazon though that would take longer time so I'm considering managing large email lists from Amazon. I have > 10 000 registered users that I never emailed and I'd like to email them a reminder that they are welcome to use my application and that they can unsubscribe from future mailings.