Most Commonly use Email in classiera.

1) New User Register: Admin and user both will receive email.

2) New Post if Pending status: Admin will receive email and will notify to approve or reject that.

3) Approve Post: If post will be approve then User will receive email. 

4) Contact to Author: There is one form on each single ads page when any visitor will fill this form and send then it will be goes to Author of that post not Admin.

5) BID: When a user will post BID then Ad Author will receive email notification.

6) Expire: When Ad will be expire then author will receive email notification.

And you can translate all email text from Translation file.