I have a large archive of email on my XP machine, all of it Eudora 7, an application that was last supported in 2006. You can still download the XP version, but the Mac download appears to be broken.
I need to migrate it to my Mac, but it's a tricky process. I don't like the odd ways OS X Mail.app handles an email database (mostly to support Spotlight search) and I fear crushing Mail.app with the archives.
So I'm researching conversion options, not for the first time. This Thunderbird kb article outlines several options. I'll be doing more research and updating this thread. I'm not particularly concerned about conserving the address book.
Once I get it converted and tested I'll look at dumping the IMAP setup I currently use with Mail.app to get my email from Gmail and possibly regress to a simple POP configuration that's less messed up by Google's peculiar folder emulation.
I'll do my test conversions with a special purpose user account I can delete after I'm done testing.