Proxmox Mail Gateway is not a ISP-Mailgateway. It is meant to be used for an internal company mailserver (like exchange). It does have an API, but it's support for multiple domains and smtp-auth is pretty limited. Better choice for ISP would be Mailcleaner or Scrollout F1. The latter has no API but it is based on csv-files, so you could export your domains+setting to scrollout. Also Scrollout has no multi tenancy features, so you could not let your customers mange their domains by themselves (only admin login which manages all domains).
