With dual delivery, users receive their mail messages in two inboxes, their legacy mail inbox and their Google Apps inbox. Incoming mail is delivered to a primary mail server, which processes and delivers each message, then forwards a copy to a secondary mail server which delivers it to the second inbox. The primary server is the mail server identified in the MX records for your public domain.

The two mail servers are your legacy mail server and the Google Apps mail server. Either mail server can be the primary mail server, depending on your needs and which legacy system you have.

• Use the legacy mail server as the primary server when the legacy server delivers local mail (that is, mail between users in your domain) differently than it delivers mail addressed to external recipients, or when you don't want to change the MX records for your domain. (Learn how to set up this configuration.)
  
  For example, Microsoft Exchange delivers internal mail directly to users' Exchange inboxes without querying the MX records for your domain, whereas it always queries the MX records for messages sent to external recipients.
  
  
• Use the Google Apps mail server as the primary server when your legacy system is a web-based or hosted mail server over which you do not have full control, or when your on-premise mail server does not support forwarding to a secondary server. (Learn how to set up this configuration.)