Google Apps Technical Transition Guide For Business, Education, and Government - Manual (Page 35)

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Chapter 6
Mail Routing
Chapter 6
Core IT
Set up dual delivery
Level of Effort: Low
Your IT team should use Gmail instead of their legacy mail client as much as possible
during this phase, which means they need to begin receiving messages in Gmail. To
accomplish this, you configure dual delivery on your legacy system. With dual
delivery, incoming messages are delivered to your legacy mail server, which in turn
forwards a copy of each message to Gmail. Setting up dual delivery has no impact on
your other users.
Note: Make sure you’ve completed Set up a subdomain for mail routing (p. 25) before
continuing with this step.