 MX records have a flow.
Well that has to do with the priority of the mx records. ie..
mail.postini1.com. mx priority 10
mail.postini2.com. mx priority 20
mail.realmailserver.com. mx priority 30
In this example mail will go to the mx10 postini.. after postini filters it passes it to the next priority mx and eventually back to the real mail server. |