i'm just in a the process of migrating a froxlor managed couriermail server to a new froxlor/debian9/postfix/dovecot mail server
there are hundreds of gigabyte imapmap emails with hundreds of accounts.
to make this a bit less painfully to move
i started to created a small email migration script for the beginning
https://github.com/veto64/sync_froxlor
requirements:
emails passwords must be plain
latest docker
remote access to the server