We're trying to develop a Shopify app but having issues finding documentation on how to send an email on behalf of the shop keeper. In fact, we aren't 100% sure how Shopify even has permission to send an email from the domain of their shops given that our own tests show no requested changes to our MX or SPF records or the like.
How are other app developers sending emails on behalf of the store owners? Does Shopify have some kind of undocumented API call to send an email from the shop's owner address? If not, are you using some third party service like Postmarkapp and, if so, how are you getting permission to send from the shop's domain?