IMO this is an API that should be implemented on your end (dnsimple), not theirs. There are a relatively small number of DNS providers which all provide essentially the same interface (CRUD operations on records), but there are a huge number of services that need to set up DNS records, including open source projects self-hosted by many individuals. It's not realistic for DNS providers to implement integration with every possible service, but the opposite is totally doable.

If dnsimple implemented something like DomainConnect, Fastmail could easily integrate. And it's a win-win because it sends traffic your way.

As I said, I found[0] DomainConnect to not be a good fit for open source projects in its current form. I've done some work on a new protocol which you can learn more about here[1]. Would be interested to hear your thoughts from the perspective of a DNS provider.

[0]: https://github.com/Domain-Connect/spec/issues/64

[1]: https://takingnames.io/blog/introducing-takingnames-io

A quick dnsimple / fastmail question, what is the purpose of the _client._smtp SRV record in you fastmail template? The only thing I can find about it seems like a draft rfc standard that was never finalized. FastMail definitely doesn't have any info on it in their support pages.