Thanks for the suggestions. All the emails are IMAP and they’re OK in webmail or other imap clients. I don’t really use much in the way of extensions, especially on the Windows system that uses the current version of Thunderbird but starting with them disabled (standard help menu feature) doesn’t help.
There’s a save feature that saves an eml file but it’s still broken when I open it in, eg, Mail.
There’s a
screenshot here