Need one cheap SMTP server, we are currently using Atomic Mail Sender.
You can use Mandrill or Mialjet. But I wouldn't recommend Mandrill becuaes they are powered by Mailchimp and they has various number of requirements and restrictions.
Mailjet will the best option for you.
I think MailGet SMTP is one of the cheapest SMTP service provider that offers highest email delivery rates, a seamless process of integration with the larger options of compatible email marketing software and a lot more productive characteristics.