As most people know there are different flavors of mail and mail servers; POP, IMAP, SMTP etc. If you have been around a while and have an ISP that uses it's own mail server, you probably have a Post Office Protocol or POP account. POP was designed for, and works best when you use only a single desktop computer. Normally, messages are downloaded to your desktop computer and then deleted from the mail server. The difficulty with POP is that all your downloaded mail resides on your computer so when you get a new machine you won't be able to access your old mail unless you move it. So how do you move mail from one machine to another?
Open the Mail application on your source machine. In the left column select the inbox and right (or control+click). From the drop down menu select "Export Mailbox..."
Save the file to your desktop. If you have created your own mailboxes on your Mac you will also need to export each one in the same way.
Once you have the file on your new Mac you will need to start the mail application on that Mac and do an import. In the "File" menu just select "Import Mailbox" for each of the mailboxes you exported.
Apple's default storage location for mail downloads is:
Since Apple doesn't normally allow you access to your library folder from the finder you will need to use the Finder "Go" menu. I bet you've never used it have you?
Open a Finder window and at the top of the screen in the "Go" menu select "Go to Folder"
Now type "~/Library/ in the "Go to folder:" window.
You don't want to overwrite any files or folders on your New Mac. All you want to do is add the contents of your old Mail Downloads folder to the Mail Downloads folder of the new Mac. Make sense?
That's it! You're done. Just as a quick FYI - You would usually want to use the Migration assistant to move your data from your old Mac to your new one. It's easy and it's what Apple recommends. But it's not always the best solution.
Sometimes it's better to do a spring cleaning and not transfer all your data and preferences. That way you can start with a clean slate and remove the chance that outdated files and old data corrupt your new machine. By choosing to move your data and files manually you get to decide what you want to keep and what you want to get rid of rather than loading down your new machine with outdated junk.