If you get a date showing 1969, it means the date header has been lost. You may be experiencing a corrupt folder.

Create new folder and move all good emails from the possibly corrupt folder into that new folder.

When original folder is empty: Right click on that folder and select 'Compact'.

• http://kb.mozillazine.org/Thunderbird_:_Tips_:_Compacting_Folders

I renamed the original folders that were giving me trouble, created new ones with the correct names, copied the contents from the old folders to the new and deleted msf etc as described. So far, so good. Let's close it and we'll see if it recurs. Thanks for your excellent advice.