I forward emails for custom domains from my registrars (Namecheap and GoDaddy) to Gmail. Then you can use a free Zoho account that lets you send email from your custom domain from within you Gmail account (sent via Zoho's email servers). It's a bit of work to set up but then you can do everything from within Gmail seamlessly.
I think this is the tutorial I followed years ago: https://medium.com/@raymondgh/how-to-set-up-a-free-custom-do...