Hi folks,
I just helped someone move 350k messages from a series of Eudora archives to Apple Mail. Anyone who says they had great luck with Apple Mail directly importing their Eudora messages is either a liar or didn't notice (or care) that their dates were all messed up. So here are some notes.
First, Eudora Mailbox Cleaner only runs under 10.6 since it's PPC and requires Rosetta. So you're gonna need a 10.6 or earlier mac (or virtual machine).
Second, After running Eudora Mailbox Cleaner, you need to poke Apple Mail into rebuilding all those mailboxes. You can go Mailbox->Rebuild for each one if you only have a handful, but an alternative is to intentionally break (aka delete) Apple Mail's mailbox cache. You can do this by deleting/renaming the file called 'Mailboxes' (IIRC, I don't have a 10.6 mac in front of me to double check). Next time you open Apple Mail it will rebuild all your local mailboxes.
Third, Eudora Mailbox Cleaner will terminate if you throw it a particularly complex (or large group of mailboxes). I assume it has something to do with the way it walks mailboxes. You're going to want to break up any mail folder that has a number of large subfolders into multiple folders or imports. So for example, if you had the following structure:
folder/
folder/a (big)
folder/b (big)
folder/c (big)
Eudora mailbox cleaner will likely crash. If you reparent your folder structure so it's something like this:
folder/part1/a
folder/part2/b
folder/part3/c
This way no individual sub folder contains folders that exceed some magical threshold. I never figured out exactly how/why this happens, but if you put your big mailboxes each in their own folder, the import will usually just work. You might also have to break up your import into smaller chunks.
A couple other things to note, Eudora allows pretty much anything in mailbox names including ~><*%$@&/\|` etc. While most of these will continue to work fine in Apple Mail, it's in your best interest to rename them to something more normalized to steer clear of trouble.
As for Lion, I never successfully got Lion Mail to import mailboxes created by Eudora Mailbox Cleaner properly. Either they wouldn't show up at all or they would show up nested with two copies of the mailbox, for example MailboxQQ had messages in it as well as subfolder MailboxQQ/Messages with a duplicate copy of the mail.
So for me, going from Eudora to Lion Apple Mail I had to do the following:
Eudora->Eudora Mailbox Cleaner->Apple Mail (on 10.6) then copy the files over to the Lion machine and let Apple Mail (on 10.7) import the 10.6 Apple Mail Mailboxes into it's new 'V2' mailbox format.
So good luck. A perfect migration is possible, it just takes some work.