Re: Exporting a mailbox over IMAP.

I don't have any direct answer to your question, but wanted to make sure you were aware of these tools, which are built to safely merge two IMAP mailboxes together with no loss. They might provide an alternative strategy.

Imapsync: https://github.com/imapsync/imapsync

Offlineimap: https://github.com/OfflineIMAP/offlineimap

P.