juznoint,
It also depends on how your .Mac account is set up on your computer. If your .Mac email account on your computer is set up as an IMAP account, then all of your messages are stored on the server and are not downloaded to your computer. So deleting a message off your computer deletes the message off the server and no other devices will be able to access it.
On the iPhone, the .Mac email preset uses IMAP, so if you delete a message off the iPhone, it will delete the message off the server, preventing it from being retrieved by your computer.
For more information on setting up your .Mac email using POP or IMAP, take a look at this article:
http://docs.info.apple.com/article.html?artnum=51729
Hope this helps,
Jennifer B.