The real integration services like Fastmail need is for DNS. It's crazy that I need to understand DNS records and copy/paste them in order to have someone else host email on my behalf. When I sign up for Fastmail, there should be a quick OAuth flow out to my domain registrar for me to delegate a [sub]domain for Fastmail to control, which returns a token they can use to set all the A, AAAA, CNAME, SPF, DKIM, etc up for me. There is a protocol[0] for such a thing, but adoption has been slow, and I found it a bad fit for open source projects.