Thunderbird Stars messages, Outlook flags them. But it is not quite that simple apparently. Outlook is essentially a corporate mail client designed from the ground up to function as a client for the Microsoft Exchange corporate mail server. In recent years Microsoft has shoe horned their exchange product into the cloud and are offing it as a part of the office365 package, unfortunately is is not designed in many way to be compliant with the internet mail standards. https://datatracker.ietf.org/doc/rfc9051/ I have not used outlook for more than a decade, so I made a trip to Google and it looks to me like this Outlook "flag" flags a message to be followed up, adds the message to the ToDo list etc. But I do not see any clear indications from Microsoft that it has any meaning outside outlook. I think this 2022 article answers your question https://www.slipstick.com/outlook/outlook-categories-flags-and-imap-accounts/ In as much as outlook saves none of the information back to the IMAP server, so Thunderbird will not be able to see it. This person which appears to have some real world experience suggests that there are problems with exchange server and setting flags in outlook. Essentially the same issue as mentioned above, but with more detail of what actually happens. https://www.reddit.com/r/exchangeserver/comments/n4l42t/imap_and_exchange_flagging_only_works_one/ It sounds to me like outlook has an issue that is not being addressed by Microsoft.

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