I've used Thunderbird for years and know half a dozen other people using it, including one who has folders with tens of thousands of emails, and have never heard of any data corruption.

The only issue with "large" mailboxes is that Thunderbird tends to become really slow. But this issue plagues other desktop clients as well.

I would love to find an actually performant email client. It shouldn't take like seconds to sort like 100 000 emails. It's a puny number. The time it takes one can read all the emails from disk a thousand times, it's sad.

I have used Thunderbird for over a decade.

I have never seen any corruption, but if I have threading on, emails get attached to the wrong thread quite often. It's quite annoying.