Faster way to import Thunderbird pop emails into dovecot imap Maildirs?

[Ian Evans]

Until recently, I was using POP on my desktop and imap to access my email
on my phone and laptop. Got tired of not having everything everywhere, so
decided to go full imap.

Followed the suggestions online and created a new imap account for my site
in Thunderbird and copied my pop folders over to the imap ones so that the
emails would get uploaded to the dovecot server and Maildir folders.

That worked for some smaller folders, but in a couple of folders, I have in
excess of 20,000 emails. Thunderbird/Dovecot seems to choke on that,
apparently timing out at some point. I've tried doing about 1000 messages
at a time, but even that is taking more than an hour and I have to be
around for each chunk.

I'm sure someone here is going to point me to some eureka moment method of
exporting the thunderbird folders to some format, sftp'ing them to the
server and importing. Just want to be careful that I don't mess up the
current batch of emails on the imap server.

Thanks for any suggestions.