ActionMailer extension works as a balancer that allows safely migrating to a new email provider, proportionally distributing the email sending load between two different sending services and keeping the sending equilibrium. For example, you set up 70% with SendGrid, and 30% with Mailtrap (or any other provider).

Check out the article for detailed info, or go directly to the ActionMailer Balancer GitHub page: https://github.com/railsware/actionmailer-balancer.