Migrating from macOS Mail Server is going to be one of the stranger migrations you might do. Why? Unless you’re moving to basically a custom build of the same tools used in macOS Server (which you’d do by forklifting /Library/Server/Mail/ into a postfix environment and putting the various components Apple changed at compile-time back together), the process for moving to a modern system is going to rely on IMAP and look a little like this: Get a list of accounts Provide the password for each account Setup an initial sync of mailbox contents Look for errors On the day that you cut MX records, do another sync On the day…