[Evolution] Old emails missing

[Ken Wright <wizard koalatyworks com>]

I'm not sure if this is an Evolution problem or a Dovecot problem.  I
run a private email server (Postfix 3.4.13, Dovecot 2.3.7.2) and, after
a recent failure and reinstall, I can't find my old emails.  Dovecot,
using imap, is pointing to the correct directory, and the old emails
are there, but for some reason Evolution (3.40.4-1) only sees the mail
that's arrived since the reinstall.

Any insights into this problem will be gratefully received!