This one is easy. I wanted to move all my existing email, including folders, filters, signatures – the whole lot, from OS X 10.5, to OS X 10.6, which was a fresh installation. I had been using Time Machine for ages, so all the necessary files were on an external disk.
TIP: Don’t load Mail before doing this. We don’t want existing preferences and accounts getting in the way.
Copy the directory ~/Library/Mail from the Time Machine to the same location on the new installation.
Copy the file com.apple.mail.plist from ~/Library/Preferences on the Time Machine to the same location on the new installation.
Load up Mail, and you’ll be prompted to complete the importation process. When that’s done, all your email will be sitting there.