I'm assuming that you're using maildir, in which case running an imap server can be pretty easy to do. I've always used courier from the default debian install

I'm assuming that you're using maildir, in which case running an imap server can be pretty easy to do. I've always used courier from the default debian install and it looks in ~/Maildir for the mails and ~/Maildir/.foldername/ for folders under the inbox and it has worked well for me before. I seem to recall the last time i tried dovecot (maybe a decade ago?) it was more difficult to setup but courier has always been nice and simple for me to manage since it's using the default PAM authentication stuff. Probably not what you'd want for an install with lots and lots of users and more complicated authentication needs but for a backup mail server it's worked great for me and also for my parents