I know this isn't a tech help blog, but I searched the web for how to do this, and couldn't find it. Turns out it's really easy, so it needs to be recorded.
I have an old laptop gathering dust that I want to decommission. I want to save five years of old emails stored in Outlook Express 6. I now exclusively use Gmail, so I want them in a format I can refer to, even decades from now when no one's ever heard of Outlook. The most standard format I've ever run across is .eml, which is not only recognized by a number of email clients, but worst-case it is text-based, so neatly viewable in Notepad.
However, Outlook Express doesn't let you do a mass-export of everything into .eml. When I tried clicking and dragging 4000 messages into a Windows Explorer folder (which works for small groups of messages), it would pop up the message, "Error copying file or folder. Cannot create or replace email: There is already a folder with the same name as the file name you updated. Specify a different name."
Since this method names the .eml files after the subject line, I presume duplicate subject names were giving it fits.
There are third-party, for-pay exporters, but I didn't feel the need to pay for these. Turns out I didn't need 'em.
I located the Outlook Express database files, inbox.dbx, etc. To find them, in Outlook Express simply click Tools > Options > Maintenance > Store Folder. Copy and paste that into the address bar of a Windows Explorer window.
Then I copied those files straight over to my desktop computer, where I'm running Windows 7.
Then I went to Microsoft's website and installed the newest version of "Outlook Express", now known as Windows Live Mail. I was careful not to install all the other Windows Live items, because I don't need all that junk.
Once installed, I opened it, and canceled the dialog box which wanted me to setup an email account. Instead, I went to File > Import Messages. I chose Outlook Express 6, and pointed it at the folder containing all the .dbx files.
This went really well. All my old messages were located in Storage Folders > Imported Folders > Inbox and so on.
This is all fine and good if I want to be strapped to Windows Live for forever, and have to go through the same confusion and trouble ten years from now when Windows 9 is out, and they call their mail program "Windows Mail Explorer" or whatever their focus groups come up with. So on to exporting to .eml.
I had been hoping Microsoft would have gotten a clue and made a .eml exporter by now, but no. So I tried what I did before in Outlook Express, only this time, it worked beautifully.
Make a folder on one of your hard drivers or in My Documents where you'd like all these to go. I'd recommend something like OldEmail\Inbox, and so on, to keep your former organization structure.
Then, with both windows in easy view, simply select all messages in Live Mail (CTRL-A), then click and drag them into the folder on your hard drive. Give it some time. It won't look like much is happening, and a simple refresh will show you an empty folder, so just wait a few minutes, then click out of that folder and back in. You should see all your messages.
Just double check it got them all by comparing the total number of messages to the total number of files in the folder. Then you're done!
There are a couple of minor problems with this method. For one, all the message names are named after the email subject line. No sender data, etc. Also the message date is missing, replaced with today's date. (Though some files randomly retained their sent date... not sure why.) For that, I'd recommend retaining the .dbx files, and the .eml files as a backup, and then when you need to look up a message, just use Windows Live. If it's not around in ten years, well at least you have the source files and the raws.