Thanks to @stephdl we have recently released a new panel to simplify the migration of email from your existing mailserver to nethserver
The module is based on the command line tool imapsync and made it even easier. With a simple interface, you can associate an email account on your existing server with the new one on NethServer.
The migration is recursive and incremental and can be repeated as many times as needed. That’s made this module very suitable and useful.
Incremental " means you can stop the transfer at any time and restart it later efficiently, without generating duplicates. " Recursive " means the complete folders hierarchy can be copied, all folders, all subfolders etc.
Take a look a this screenshot 
You can find more info on our official documentation
https://nethserver.docs.nethesis.it/en/v7/mail.html#imap-synchronization