This used to be such a headache, but now it’s pretty simple, providing you take a few simple steps, you can be up and transferred in no time at all.
A good piece of advice is to always clean up your email before you do a transfer, if you only have 10 – 20Mb of data, then don’t bother – but if your like me and have a 2Gb+ mailbox, a bit of spring cleaning never foes amiss. Simple things – just remove any unwanted inbox items, get rid of those old emails that you have sent, clean the junk mail folder and lastly empty the trash (unless you really want to take it with you) – with any luck your mailbox will now be smaller (saved me about 400Mb).
Fish out your mail files on the Tiger machine, go and visit [Username]/Library/Mail/ and [Username]/Library/Mail Downloads where the [Username] is the name of your account.
Copy all of this information to a CD, HDD or a network location accessible by the new machine. Lastly, visit [Username]/Library/Preferences/and find the com.apple.mail.plist and copy this too.
Now on your new machine, copy all of the files into their respective locations, they are the same locations on a Leopard machine – run mail, and you will probably be asked for the passwords for your account. You can get around this by copying over your keychain from the old system – but that is a conversation for another time.