networking and email

I have 3 computers on my network and use a Linksys wireless-G router.  2 of the computers run winxp and the other is a macpro laptop.  I have always been able to get email on both the main computer and the laptop..  after adding the 3rd computer to the network, all of our emails stopped coming to the main computer and laptop and now go to the 3rd computer...  what can i do to get them coming in on all computers again??