I'm going to take a wild guess that you have been using your desktop for mail, and now you've got a laptop, and you want to use that for mail instead.
The very simplest way of doing this is to keep your desktop, but tell it not to download mail anymore (e.g. change the mail server name to 'localhost'). Once you've done that, you can set up the mail account on the laptop, and go to the desktop if you need to refer to old mail.
If you want to copy the mail from the old computer to the new one, that depends very much on the mail software you are using -- the very easiest way of doing this is to not do it. Your favourite search engine should be able to tell you how to do it for your software - the magic words are "backup and restore" or "migrate":