I tested with imapsync to get an initial sync - worked for me.
This shall be operated once only to get an initial sync - after this fetchmail does the rest of the daily job.
I moved a total of 16T e-mails using this tool. Took me a while
P.S. Note that you cannot use the --sslx option for IP
imapsync --host1 server1 --user1 user@server1 --password1 user1password --ssl1 --host2 server2 --user2 user@server2 --password2 user2password --ssl2
P.S. 2: What about this tool: https://stash.kopano.io/projects/KC/repos/kopanocore/browse/ECtools/migration-imap/kopano-migration-imap - seems like the same based on imapsync