Thunderbird works with mbox files. It can use the maildir format too, but some people believe that it may not be reliable yet. Those are the only two formats in Thunderbird.

Third-party programs can convert Microsoft messages to mbox format.

Working off-line in Thunderbird can be done.

All IMAP accounts sync with a server. If you use an IMAP account in Thunderbird, it will sync with a server too. Is Microsoft requiring something more with syncing IMAP “to the cloud”?

Thank you, Rick, and sorry for the slow reply. I don't know of anything else that MS is requiring. I'm not too tech savy, so there may be more to it. I'm not a big user of cloud computing, mainly because I don't understand it and don't trust that my data won't be hacked. Maybe my options are limited. Thanks, again.

I understand now what Microsoft is doing. Apparently, there is a way to keep using "Classic Outlook":

https://learn.microsoft.com/en-us/answers/questions/4736051/why-wont-new-outlook-for-windows-let-me-configure

I don't know anything about Microsoft's plans, but I can imagine that the option to keep using Classic Outlook will not remain indefinitely.

Do you have a reason not to trust Microsoft's security practices? If you do, you may want to consider using encryption with Outlook. Working off line may not help much because mail still has to go through a server. Maybe you just want to move messages to local folders so they are not hackable on a server?

Apparently, your IMAP account is hosted by someone other than Microsoft? If so, in your place, I would switch to Thunderbird. Switching is easy. Converting any messages stored in local folders will take some effort.

You may also want to consider encryption, which can be done in Outlook or Thunderbird. You could also switch to a a service that always encrypts everything, such as Proton Mail.

People here can help.

Good luck.