When traveling, especially with a mobile device, you might want to avoid downloading spam messages from the server. If your server and mobile device support IMAP or Exchange, you can do this by following the instructions in the Using a Spam Mailbox on the Server section.
If your mobile device only supports POP, you can do this by letting your Mac at home filter the messages from your main mail account and redirect the non-spam messages to the account that you access when traveling. The spam messages will stay in the Junk mailbox on your Mac.
Open Mail’s Settings window and click the Add Rule button. Enter a description, such as “Redirect good messages to handheld.” Change the condition from From to Every message. Then change the action from Move Message to Redirect Message and enter the e-mail address of the account that you check when traveling. Click OK. Finally, drag your newly created rule so that it is just below the SpamSieve rule.