Applies to: Exchange Server 2007 SP3, Exchange Server 2007 SP2, Exchange Server 2007 SP1, Exchange Server 2007
Topic Last Modified: 2006-09-08

Before you create an address rewrite entry that rewrites multiple domains, you must prepare your sub-domains. Also, before you perform any of the procedures that are described in How to Rewrite All E-Mail Messages from Sub-Domains, you must understand the requirements for rewriting e-mail addresses in multiple domains to a single domain, and the appropriate pre-configuration for the affected mailboxes and contacts.

Important Considerations

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, there cannot be a

  • 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 is rewritten to, the e-mail address 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 is rewritten to, you must create a new mail-enabled contact in Active Directory with as the target SMTP address and 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.

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.

Removing Conflicting E-Mail Addresses

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, and







Each sub-domain has two users, and each user has a unique e-mail address. However, you want to rewrite the subdomains,, and into a single domain that is called 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

Original e-mail address Rewritten e-mail address

When the e-mail addresses in each sub-domain are rewritten, a conflict occurs between and, as indicated by the bold text in Table 1. Therefore, both e-mail addresses are rewritten to 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.

Applying Proxy Addresses to Recipient Mailboxes

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, and the rewritten external address is, the mailbox must be configured by using a proxy address that is set to