How do users can login automatically through URL when they click on the link received in Email
Try the Login one time module


  Login one time provides the ability to email one-time login links to users. You can also choose the pages they will land on.


Another module is urllogin


  This module provides a way to transparently log users into the web site when they follow a link. The primary use is for mass email where users are send individualized URL’s which take them to a page on the site, in a “logged in” state. Every effort has been made to minimize the potential security risks.