I have years of email messages locked up in a 1.2 GB Eudora for Windows archive . Converting them has been a pending project for about 10 years. This archive is a part of my extended memory; it has the keys to remembering people and places now forgotten.
Now time is running out. Eudora Mailbox Cleaner is one of the best tools for this project, and it's PPC only. Since Lion doesn't have Rosetta, the app won't run on 10.7 or higher. 
I've downloaded a copy. I think I'll delay my primary machine's Lion conversion until the Eudora conversion is done.
There are other Eudora archive conversion methods, but most run on Windows -- and I think Windows VMs on Mac will fade away as Apple locks the OS down to both block malware and enforce DRM. Time is passing. If you have old archives, you may have only a year or so to (relatively) easily convert them.
I'm planning to create a user account to test the import, then backup up my current Mail database prior to adding this one.
- Gordon's Tech: Converting Eudora email
- Gordon's Tech: Converting Email from Eudora OS X to a modern format - the TidBITS review: Adam Engst's comprehensive guide
It took a few hours to do the conversion; most of that was spent waiting for a bundled AppleScript to Rebuild Mailboxes. Post conversion the database is 2.4 GB and over 55,000 messages.
Follow the directions carefully. I didn't think the nicknames were worth converting until I saw them; the connections between Groups and Members turned out to have powerful memory value. I recommend testing first with a special user and an empty address book and email account. If you import into an existing Mail database be sure to backup ~/Library/Mail first.
I didn't import Filters. They'd just be noise now.
With OS 10.4 and later there's an essential 'Mailbox Rebuild' step. My Eudora had at least 478 folders across a deep hierarchy . It would have been extremely tedious to try to rebuild them all. So I tried the 10.4-10.5 rebuild AppleScript, even though I am running 10.7 Snow Leopard.
That script runs slowly, and because of the way UI Scripting work it must own the entire machine. It will steal focus. Don't try to even switch users. It took over an hour to complete. Thank Darwin it's there; the job would have been untenable without it.
As per the documentation (mandatory read):
Your imported mailboxes will be located in a new folder called "Import" at the top-level of your local mailboxes. The new files will be located at "~/Library/Mail/Mailboxes/Import" on your hard disk...
... due to the way the Address Book works, a new group will be created for each Eudora nickname which expands to more than one email address.
I've already leveraged the archive to rebuild memories of people I worked and corresponded with in the 1990s. Some have died, some I've lost touch with. My old Eudora email archive has now been reintegrated into my extended memory.
For now the email archive and my old Address Book are sitting in a distinct user account. I will decide over the next week whether I'm going to leave them there or, more likely, repeat the process. If I do a complete merge I may create yet another user account, copy both my current email archive and this archive there, and let the process run.
OS X mail has a virtually undocumented import Apple Mail Data feature. I don't think it will be practical to import the archive using that but I'll take a look at it. (Update: It seems to have worked quite well. I imported my old Netscape emails at the same time.)
- fn -
 A year or two ago our 8 yo G5 iMac hard drive died. I dithered about whether to replace it or not because of the infamous iMac capacitor problem and its slowly delaminating screen. (The G5 iMac was one of Apple's most troublesome machines -- they desperately needed to move to Intel. The heat problems were extreme. It was, however, extremely easy to repair. That was even a marketing feature. Perhaps the G5 iMac is why Apple doesn't design for fixability any more.)
I did replace the drive and it's still working. It can even, if I were to install 10.4, run Classic. It's now one of my most valued machines. I think I may turn our MacBook into the kid machine and put the G5 iMac on a very light duty cycle so I can keep it available for another decade. I'll also archive my software install library onto its now capacious hard drive.
 My Gmail archive has essentially 2 mailboxes: In and All. Full text indexing and search changes things.
 Created by Steve Dorner, Eudora was the preeminent Mac email software and was dominant for a time on Windows as well. This archive, during its lifetime, moved from Mac to Windows. I found some 1996 correspondence with Steve, he was a very approachable developer with a lot of correspondence!
Before Eudora I used Norton Commander Mail (ncmail.exe) with MCIMail (pre-inernet -- firstname.lastname@example.org was my email at the time.
Writing this post inspired me to look into those archives. I did have them -- zipped up from the days when 200K was a lot of data. OS X couldn't open the old zip archives, but Unarchiver did fine. I used Automator to attach a .txt extension to each file and assigned them to TextWrangler. That was enough to convince OS X to index my old NC email messages. For example:
I actually got a bunch of public domain and shareware Windows applications running SLIP under OS/2 over my 14.4K modem ...
... The second thing I'm excited about ...) is learning HTML and creating my own home page on the SILS Mosaic connection...
... for my next 'HOT tip'. Check out TeleScript. Ignore "General Magic" -- it's bogus. TeleScript is the secret pearl.
I didn't remember that 14.4K modems coexisted with HTML 1.x. TeleScript is huge in an alternative reality. GM was a copy co-founded by "Bill Atkinson, Andy Hertzfeld and Marc Porat". Historic names.
The next archive to crack is a set of email written using Netscape's email app. (Update: Mail imports these directly.)