It shall be a really good feature because many website owners use subdomains for email delivery.
RSForm has a great tool - SQL mappings which allows submitting the form data into external database and it works, there is the problem with "id" and "key" fields in "btqfv_acym_user" table, The key seems to be a random alphanumeric number, but I don't understand how user's ID is formed, it should be an array value.