The Mail application on your MAC has a built-in engine to determine which message is spam and which is not. Each time you mark e-mails as spam or not, your application gets "smarter" and will filter your mails better. This can be a long process and if you have spent some time doing that, it could be inconvenient to start the process all over again if you have to use a new MAC.
Therefore, you can copy the settings from your "trained" MAC to the new one. On the computer that you have already specified your mail rules in, you should go to your userfolder/Library/Mail and find the LSMMap2 file. In this file, Mail stores the junk mail training data.
On your new MAC, you should first quit Mail. Then navigate to the same folder and replace the current LSMMap2 file with the one from your other computer. You can use any available transportation means for the transfer, from a local network to e-mail. When you have the file moved, you should start the Mail application. Now it will have the same junk mail settings as on your previous MAC.
