Thunderbird does not "do" outlook style flags. Thunderbird has stars. The standard has stars. Outlook appears to use the stared and flagged interchangeably. https://support.microsoft.com/en-au/office/how-do-i-sort-my-inbox-by-unread-flagged-starred-or-attachments-9d91409a-c26d-4c29-900d-d760387e7c8f So I guess the importer will import mail as starred if it is flagged, but as I do not really do anything with Outlook and have not done so for more than 20 years I am no expert.