Apologies for my badly spelt initial question. I have now answered it myself. The basic process: # Make sure Apple Mail is closed. # Install Thunderbird. Make sure it is completely synchronised with your IMAP account. # Install the excellent ImportExportTools by Paolo Kaosmos. # Export the entire mailbox as an MBOX file. # Use Thunderbird to move the messages from the IMAP server to your local computer. # Make sure the IMAP folder is empty. # Close Thunderbird # Launch Apple Mail. Make sure it is completely synchronised. # Import the MBOX file. # Drag the contents of the import folder on Apple Mail on to the IMAP folder. Please note, Apple Mail may well say Moving one message of x thousand for ages (days in fact) while it tries to work out WTF it is doing, but eventually it will correctly upload everything and even parse out the attachments properly so they display correctly in the column view as well as inline.