This major release includes three major changes:

1. it introduces a new metadata format,
2. email flags are now backed up,
3. it introduces a 'mirror' command.

Below is the information form the CHANGELOG.

New Metadata Format

• Introduced a new metadata format.
• Included data about message offsets and lengths in the new metadata to
  facilitate for future integrity checks.
• Added a migrator to transform the old (version 2) metadata files
  into the newer (version 3) files.

Flags Backup

• Added backup and restore of IMAP flags (e.g. "Seen", "Draft").

Mirror

• Added a 'mirror mode' to account configuration that changes backup behaviour:
  • removes local folders that are no longer to be backed up,
  • removes emails that are no longer present on the server,
  • updates flags on the local backup to match those on the server.
• Added a 'mirror' command that takes a 'mirror mode' backup and copies
  it to another server.
• Added a '--refresh' option to the backup command that, updates flags
  on the local backup to match those on the server, even for accounts
  that are not in 'mirror mode'.