Today is a good day. It turns out that this is now available, but just a bit hidden. I've just authenticated our domain and confirmed that emails are being signed by our domain!
Hip, hip, horray!
Further reading, that "Learn more" link goes to here, where they have added some documentation about it:
Thanks again Sebastian for breaking the news!
Is everyone's authentication still working? Ours seems to have stopped signing emails from our domain a couple of days ago, even though it is still showing as authenticated. This might coincide with a change in our DMARC policy (I don't see why that should be the case though). I am in touch with Shopify support to try to get to the bottom of it. Frustrating!
Hi @Cookies, I got in touch with Shopify support and today it was escalated to those responsible for DKIM. Now it is working once again. I'm interested to know if they have fixed an issue that was clearly affecting others, or whether they've just fixed us. Let me know!