If the source server does not have one of the supported hosting
platforms installed, you can use custom migration to
migrate from a server running an unsupported hosting platform, or no
hosting platform at all. Custom migration involves creating a hosting
description file written in YAML or JSON and detailing the objects
(domains, mailboxes, customer accounts, and so on) to be migrated.
Custom migration can be performed even if you do not have administrative
access to the source server.
Note: You can only migrate from custom hosting via the command line.