If your intent is switching to a different email client, you're probably best served to keep with whichever format you now use, whether POP or IMAP. Thunderbird is well supported and there are lots of help articles on web for the transition. Your simplest import would be to download an older version of Thunderbird, one active 5 or 6 years ago because its import feature supported Outlook Express. There is a nice article here, complete with graphics: https://www.datarecovery.institute/transfer-outlook-express-messages-to-thunderbird/ and old versions are at https://archive.mozilla.org/pub/thunderbird/releases/ I think version 15 or 17 may work, but that's my guess. If you do that, I suggest doing a full backup and then update to current version. I have not done this, but it's doable. edit: removed unofficial third-party download site link.