Google: Dual Delivery and Split Delivery for Google Apps

If you’re using multiple mail servers, it may be important to note the distinction between dual delivery and split delivery.

In dual delivery, a copy of every message is sent to two separate servers. On a successful dual delivery, both mail servers receive a copy of a message. You can implement dual delivery in Delivery Manager using these instructions.

In split delivery, incoming mail is routed to one of two servers, based on your user settings. On a successful split delivery, only one mail server receives the message. Which server receives the message is based on the address of the recipient. Set up split delivery by creating multiple email config organizations, setting Delivery Manager for each email config to route to the appropriate server, and adding appropriate users to the user orgs under these email configs.

You can use dual delivery and split delivery together, but plan your delivery settings carefully. For instance, you could set up two separate email configs to use split delivery, and then set up Dual Delivery on each email config, in order to deliver a copy of the message to Gmail.

For more information regarding dual delivery, go to https://support.google.com/a/answer/173534?hl=en

For more information regarding split delivery, go to https://support.google.com/a/answer/173534?hl=en


Reference:
Google Postini Services
https://www.google.com/support/enterprise/static/postini/docs/admin/en/admin_ee_cu/deliv_dual.html