Freebie Outlook Express Eml Importer was written for the express purpose (pun intended) of automating the import into Outlook Express of .eml e-mail files exported from Thunderbird by IMAPSize, or the more automated Tbird2OE. It worked perfectly in my testing, and was fast--importing and recreating the folder structure of about 27,000 e-mails in less than five minutes. It took Tbird2OE about two hours to generate those individual .eml files in the first place.
Outlook Express Eml Importer's interface is as simple as can be: one field for browsing and locating the .eml files, one field where you can name the folder you want sent e-mails stored, and an Import button to commence the process. There's also a small display list that tells you what's being and has been imported. If I have any gripes, they're that the import path field should be labeled Path to Eml files instead of just Path, and that (as most users likely to use the program only once) a portable version that doesn't require installation might be in order.
These piddling complaints aside, Outlook Express Eml Importer is clean, simple, and free--and it works. If you're switching back to Outlook Express or moving to Outlook (Outlook imports from OE, but not Thunderbird) then this program, along with the aforementioned export utilities, will save you a ton of time and effort.