[walrus01]

1) you will need to point the DNS MX records to office365 or gsuite or fastmail or any other service

2) sign up with that service and input the domain name, do the rest of the configuration.

There is a huge blurry overlap between domain registrar and hosting services these days. It sounds like you are using Namecheap for both. I would highly recommend using domain registrars as ONLY domain registrars and having other things hosted elsewhere.