Constant Contact is a 3rd party service that sends email for you.

How to setup -

https://knowledgebase.constantcontact.com/articles/KnowledgeBase/18053-activating-and-installing-your-sign-up-form?ic=comm_reply&lang=en_US

Landing Page

1. Login to Constant Contact account.
    
2. Click on 'Sign-up Forms'
   
   
3. Add a new form. Click on Landing Page.
   
   
    
4.  

Pop-up Form

1. Login to Constant Contact
    
2. Click on 'Sign-up Forms'
   
    
3. Add a new form, click 'Pop-up'
   
    
4.