>... The one holdout is Mozilla Thunderbird. It stores data in two files per email folder. I can see that they correspond to the email folders I had before but are stored in an inscrutable format I haven't been able to crack yet.

Dragan already pointed you in the right direction - although I'd do it backwards - copy the whole folder to somewhere in your Dir structure easy to see, backup and out of C:\ and then point the new Thunderbird to that directory - I am lazy ;-)
They are not inscrutable - you can read most stuff in a hurry as ASCII with some mishmash and AFAIK it is SQLite ;-)

> I tend to retain only emails that are worth saving so would really like to get them back.

Year end tasks here include moving 2Y old mails into "old" folders, which grow once a year and are somewhat hefty in size. Then they are saved right away, so that for the next 12 months the newly arrived mails are in smaller folder/files (13 months worth in Jan, 24 in Dec) which can be backupped faster. Searching still easy if you organize the folder structure to easily drill down.

regards

thomas