Woo-hoo! Found a bug in #macOS 26 (Tahoe) Mail.app with TLS 1.3: if you set up your IMAP server to only speak TLS 1.3 then Mail on macOS 26 just hangs whereas it works fine on iOS 26…
There is an (old) discussion from 2022 which was replied to by an idiot saying "Apple says it does, so it must be true."¹ 
The fact that exactly the same Dovecot server works fine with TLS 1.3 enforced against iOS 26 w/ Mail but does not against macOS 26 Tahoe w/ Mail is pretty damning to me...
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¹ https://discussions.apple.com/thread/254448705?sortBy=rank