When you flatten internal sub-domains into a single external domain, you must consider the following factors, which apply only when multiple sub-domains are being rewritten:

• Unique aliases are required   All e-mail aliases, the part of the e-mail address to the left of the @ symbol, must be unique across all sub-domains. For example, if there is a joe@sales.contoso.com, there cannot be a joe@marketing.contoso.com.
  
  
• Proxy addresses are required   A proxy address that matches the e-mail address that is produced by the Address Rewriting agent must be configured on every e-mail account that is in the sub-domains that are rewritten. For example, if joe@sales.contoso.com is rewritten to joe@contoso.com, the e-mail address joe@contoso.com must be added as a proxy address to Joe's mailbox.
  
  
• Contacts may be required   If you are rewriting e-mail from a non-Exchange Server 2007 e-mail system, you must create Active Directory mail-enabled contacts for each e-mail address in the non-Exchange Server 2007 e-mail address that is being rewritten. This mail-enabled contact must contain the original e-mail address and the rewritten e-mail address. For example, if joe@unix.contoso.com is rewritten to joe@contoso.com, you must create a new mail-enabled contact in Active Directory with joe@unix.contoso.com as the target SMTP address and joe@contoso.com as the proxy SMTP Address.
  
  

These factors are important because rewriting addresses in multiple sub-domains causes a many-to-one relationship between internal sub-domains and the externally visible domain. Because of this many-to-one relationship, the Address Rewriting agent cannot determine which sub-domain contains the correct recipient when a message that is addressed to the externally visible domain is received.

Important:
Make sure that every e-mail alias that exists across all sub-domains is unique. Exchange Server 2007 does not check to verify that every e-mail alias that can be rewritten to a single domain is unique.

To create an address rewrite entry that rewrites multiple sub-domains, you must first make sure that all e-mail aliases are unique across all your sub-domains. For example, consider the following configuration:

The following users are in the subdomains sales.contoso.com, marketing.contoso.com and research.contoso.com:

• maria@sales.contoso.com
  
  
• chris@sales.contoso.com
  
  
• david@marketing.contoso.com
  
  
• brian@marketing.contoso.com
  
  
• chris@research.contoso.com
  
  
• carter@research.contoso.com
  
  

Each sub-domain has two users, and each user has a unique e-mail address. However, you want to rewrite the subdomains sales.contoso.com, marketing.contoso.com, and research.contoso.com into a single domain that is called contoso.com. Table 1 shows each original e-mail address and its corresponding rewritten e-mail address.

Table 1   Original e-mail addresses and corresponding rewritten e-mail addresses

When the e-mail addresses in each sub-domain are rewritten, a conflict occurs between chris@sales.contoso.com and chris@research.contoso.com, as indicated by the bold text in Table 1. Therefore, both e-mail addresses are rewritten to chris@contoso.com. To resolve this situation, you must change the e-mail address of one of the recipient mailboxes to an address that does not conflict with the e-mail address in any other sub-domain.

For internal recipient mailboxes to receive replies to addresses that have been rewritten, you must configure those recipient mailboxes by using a proxy address that matches the rewritten external address.

For example, if a mailbox exists for carter@research.contoso.com, and the rewritten external address is carter@contoso.com, the mailbox must be configured by using a proxy address that is set to carter@contoso.com.