Thanks all for this thread. I had the same problem and a few old mailboxes. In the end, I automated the recovery as suggested by ellie timoney on March 25, 2022 (thank you ellie!).
A gist is here: https://gist.github.com/larsimmisch/f0c02977ffc9a9c72781716eaa3d7334
Maybe it will help somebody else with the same problem. But be careful! It worked for me but it's otherwise untested.
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Cyrus: Info
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