Accepting inbound mail for a DNS domain and relaying it to responsible mail hosts outside your Exchange organization was a simple process in Exchange 2003, albeit one that required some explanation, which was discussed in the following articles:
Exchange 2003 used Recipient Policies to define the domains for which your Exchange servers will accept email and to generate email addresses for your recipients using those domains.
Additionally, Recipient Policies also allowed you to apply Mailbox Manager settings, the Exchange 2003 equivalent of Messaging Retention Management (MRM).
Read complete blog at http://blogs.technet.com/b/exchange/archive/2011/10/07/accepted-domains-shared-smtp-address-spaces-and-recipient-filtering.aspx
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