Error: Mailbox isn't a valid mbox file
This error is caused by a well-known bug in the IMAP implementation used by your email server. This typically affects IMAP servers known as DoveCot. Some DoveCot servers indicate that special folders can be enumerated, but fail with this error when MigrationWiz attempts to retrieve content. For example, we've seen this affect some mailboxes containing a "Mail/dovecot-uidlist" system folder.
As part of the IMAP protocol, an IMAP server can indicate whether a folder can be enumerated or not. MigrationWiz only retrieves content from folders marked as enumerable. If a folder marked as enumerable cannot be accessed, MigrationWiz considers the migration as failed. Folders may contain a large number of items, so we believe it is best to report any failure to enumerate content from an entire folder as fatal.
Upgrade Your IMAP Server
Consider upgrading your IMAP server to a newer version, with proper reporting of folders as enumerable vs. non-enumerable.
Filter Failing Folders
If your IMAP server cannot be upgraded, you may filter out the folders that cause the migration to fail.