From my previous comment:
Settings are imported, but no mail folders, nor the entire folder which is found in TB (I imported settings from an unused TB folder). I first had to enter all the passwords, but no mail was downloaded until I restarted Messenger. And, only mail which was still on server.
I, then went to the profile from which settings were imported, and copied a couple of files, and pasted them in the correct place. No Sent mail was imported, but all six address books were imported ok.
Now, I heard that people who first set up Thunderbird got the settings, but not the folders. I had no experience with that, as I, always, set up brand new accounts, and got my mail which was on server. This time I went through the same process that was followed by other people, and found that no mail was imported, although the settings were. This has nothing to do with Messenger being alpha. Everything was ok after I saved the passwords, and downloaded mail on server. As for older mail, I had to rename the Inbox, and copy the renamed file, and paste it in the appropriate folder. I may be wrong but I saw the same behavior with Thunderbird.
I think that there are a few posts to this effect.
Now, if anyone set up Thunderbird, and got all folders, with its mail, I will have to say that it may have something to do with NS9MS being alpha.
The fact remains that while mail from NS4.x, Outlook, Outlook Express, and Eudora can be imported, NS6/7, Mozilla, Seamonkey, and Thunderbird cannot be imported.
I, even, sent a feedback to Netscape, saying that it would make NS9MS superior if it could import everything with one keystroke.
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.9) Gecko/20071025 Firefox/2.0.0.9