Thunderbird does not yet support Exchange, but will soon. For now, you may need an addon, either OWL or EXQUILLA. I think there is a 30-day free trial and a modest annual fee. Not sure if this is what you need but it may help. A few years ago, I had an exchange account and used exquilla with excellent results.

Thanks for the reply, David. I have Owl but haven't been able to uncover my tree! Looks like I may have to resort to Outlook - I suppose that a bit of use will result in familiarity and comfort/confidence. But I really want to stay with Thunderbird.

I am only a volunteer, so don't know all the issues, but support for exchange was anticipated to be here, but was delayed for further testing. Check announcements periodically for updates. Here is latest update: https://blog.thunderbird.net/2024/04/adventures-in-rust-bringing-exchange-support-to-thunderbird/