Whenever new users are added, they will have a state as "New User".
Similar to Opt-In, we will have the Opt-Out mail sending feature, which will be by default sent to "New Users" only.
Once the email is sent "New Users", convert status to "Subscribed" & set subcribed_at time.
In the email, we will have link to "Unsubscribe", which will behave as it is. In addition to that on un-subscribing, we need to maintain timestamp on the "unsubscribed_at" field.