@John Siciliano & @Mr Bean, I assume (John), your're referring to Migadu the email service that I use?
https://migadu.com/ It's pretty simple and cheap to get going. The docs are simple to follow and setting up records in Cloudflare is super simple. This is a decent option for having unlimited customers. I haven't hit any limits on my cheap tier in the past year.
For a registrar, I can't solely use Cloudflare as my websites are often Canadian and they don't have .ca domains (our gov getting their nose in everything making it more difficult). So, I often use namecheap then have cloudflare handle the DNS records.