Courier to Dovecot Maintenance
The conversion process may result in POP/IMAP being down for a few minutes on your server, and during this process you may experience temporary difficulties accessing your email.
Email being re-downloaded
In rare occasions, your mail client may attempt to re-download email that you have already downloaded or marked as read. In cases we have seen, this will occur when you are using POP in your mail client with the option to keep email stored on the server. This is a one-time occurrence and should not continue to be an issue once the email has been re-downloaded.
Misc IMAP errors
More commonly with Outlook, you may see a problem where your mail client will show a non-descriptive error when attempting to download or synchronize mail folders. This is usually resolved by closing and re-opening your mail client.
Any additional folders you have created will be maintained during the conversion. In some cases, your mail client may not see them. If this is the case, you simply need to resubscribe to the additional folders:
How to subscribe to specific IMAP folders in Outlook 2010