I've used imapsync in the past to migrate thousands of mail accounts from and old decrepit IMAP environment (with some weird edge cases) to a shiny new IMAP bas

I've used imapsync in the past to migrate thousands of mail accounts from and old decrepit IMAP environment (with some weird edge cases) to a shiny new IMAP based mail platform. It's a hugely flexible and versatile tool and it's quite zippy as well, can highly recommend.