Importing sent e-mails as .mbox into Mail.app

After years of putting it off, I decided to finally dump Thunderbird and start using Apple's homegrown Mail app. I had tried multiple methods of getting my e-mails out of Thunderbrid and into Mail but - after many false starts and missing e-mails - I settled on simply selecting all messages in a given folder (inbox, sent) and saving as an .mbox file. Mail's own import tool never worked correctly for me, presumably because I have multiple e-mail accounts that have been merged, transfered between computers and operating systems, and tinkered with so much that Thunderbird's profile folder structure is somehow differing from the norm.

At any rate, I imported my inboxes (in .mbox format) into Mail and it worked beautifully! All the messages were marked as unread, but they were all present and accounted for and I was quite happy.

When I tried the exact same process (selecting all messages and saving as .mbox in Thunderbird, then importing into Mail) for my Sent folder, however, I encountered a problem. Mail only imported a single sent e-mail (the most recent one)! This happened with multiple attempts at multiple separate sent folders. I tried "Get Info" on the .mbox files, and - seeing as they were several hundred MB - they clearly contain more than that single message. This leads me to suspect that the issue isn't with corrupt files or Thunderbird saving them incorrectly, but rather an issue with the way Sent e-mails are handled or read by Mail.

Can anyone explain exactly what is causing this or offer advice on a way to move my sent e-mails over? I am able to save in EML, HTML, Plan Text, and other formats, though it appears that Mail really wants .mbox or a specific file from another app (once again, the Thunderbird import option doesn't work for me, so I am unable to rely on that).

Find my details below:

iMac running OSX 10.8.2

Thunderbird 17.0.2

Mail 6.2

Thanks in advance for any insight!