As an addition to this issue: https://www.virtualmin.com/node/68967
We have DNS managed somewhere else (with fallback on different locations etc), so we cannot use the default integration. What I'm looking for is a way to add custom domains through the api.
I'm now manually putting them in through "Email Settings" -> "DomainKeys Identified Mail" -> "Domains to sign for" (technical name: extra).
It would be nice if there would be an API for that.