ActionMailer Balancer: Keeping the Email Sending Equilibrium (github.com)

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).

You can check out the article for detailed info: https://mailtrap.io/blog/actionmailer-balancer/