If old mailboxes aren't that big (500.000 items max), Office 365 has a built-in IMAP migration tool.
Let it run over the weekend, and the only thing needed to transfer to PST are calendar, contacts, notes, etc
Copying back PST over slow internet is error prone, and there's no resume function, making it extremely difficult to do manually.
Microsoft Outlook is a personal information manager from Microsoft, available as a part of the Microsoft Office suite. Although often used mainly as an email application, it also includes a calendar, task manager, contact manager, note-taker, journal, and web browser.
TRUSTED BY
It's a bit terse to work with.
It's terseness is made up for by it's flexibility.
This tool is backing store agnostic, so no consideration if your using Exchange or Dovecot or Gmail.
So long at all servers involve speak IMAP4, this tool works well.