Migrating from Custom Hosting
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.