https://simplyian.com/2015/01/07/Hacking-GMail-to-use-custom-domains-for-free/
Mailgun advertises itself as a set of “powerful APIs that enable you to send, receive and track email effortlessly.” Reading that description, you may be wondering how a developer tool could allow you to use Gmail with custom domains for free.Basically, Mailgun has two components that allow you to do this: an email forwarding service and an SMTP server.
I’m using Mailgun for several email accounts that I own and can vouch for the method outlined on the simplyian site.