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Address Rewriting on Edge Transport Servers

 

Applies to: Exchange Server 2013

Topic Last Modified: 2014-02-10

Address rewriting modifies email addresses of senders and recipients in messages that enter or leave your organization through an Edge Transport server. Two transport agents on the Edge Transport server provide the rewriting functionality: the Address Rewriting Inbound Agent and the Address Rewriting Outbound Agent. The primary reason for address rewriting on outbound messages is to present a single, consistent email domain to external recipients. The primary reason for address rewriting on inbound messages is to deliver messages to the correct recipient.

The address rewrite entry, which you create, specifies the internal addresses (the email addresses you want to change) and the external addresses (the final email addresses you want). You can specify whether email addresses are rewritten in inbound and outbound messages, or in outbound messages only. You can create address writing entries for a single user (chris@contoso.com to support@contoso.com), all users in a single domain (contoso.com to fabrikam.com), or for users in multiple subdomains with exceptions (*.fabrikam.com to contoso.com, except legal.fabrikam.com).

ImportantImportant:
Regardless of how you plan to use address rewriting, you need to verify that the resulting email addresses are unique in your organization so you don't end up with duplicates. This is because address rewriting doesn't verify the uniqueness of a rewritten email address.

Contents

Address rewriting scenarios

Message properties modified by address rewriting

What address rewriting doesn't change

Considerations for outbound-only address rewriting

Considerations for inbound and outbound address rewriting

Considerations for rewriting addresses in multiple domains

Priority of address rewrite entries

Digitally signed, encrypted, and rights-protected messages

The following scenarios are examples of how you can use address rewriting:

  • Group consolidation   Some organizations segment their internal businesses into separate domains that are based on business or technical requirements. This configuration can cause email messages to appear as if they come from separate groups or even separate organizations.

    The following example shows how an organization, Contoso, Ltd., can hide its internal subdomains from external recipients:

    • Outbound messages from the northamerica.contoso.com, europe.contoso.com, and asia.contoso.com domains are rewritten so they appear to originate from a single contoso.com domain. All messages are rewritten as they pass through Edge Transport servers that provide SMTP connectivity between the whole organization and the Internet.

    • Inbound messages to contoso.com recipients are relayed by the Edge Transport server to a Mailbox server. The message is delivered to the correct recipient based on the proxy address that's configured on the recipient's mailbox.

  • Mergers and acquisitions   An acquired company might continue to run as a separate business, but you can use address rewriting to make the two organizations appear as if they're one integrated organization.

    The following example shows how Contoso, Ltd. can hide the email domain of the newly acquired company, Fourth Coffee:

    • Contoso, Ltd. wants all outbound messages from Fourth Coffee's Exchange organization to appear as if they originate from contoso.com. All messages from both organizations are sent through the Edge Transport servers at Contoso, Ltd., where email messages are rewritten from user@fourthcoffee.com to user@contoso.com.

    • Inbound messages to user@contoso.com are rewritten and routed to user@fourthcoffee.com mailboxes. Inbound messages that are sent to user@fourthcoffee.com are routed directly to Fourth Coffee's email servers.

  • Partners   Many organizations use external partners to provide services for their customers, other organizations, or their own organization. To avoid confusion, the organization might replace the email domain of the partner organization with its own email domain.

    The following example shows how Contoso, Ltd. can hide a partner's email domain:

    • Contoso, Ltd. provides support for the larger Wingtip Toys organization. Wingtip Toys wants a unified email experience for its customers, and it requires all messages from support personnel at Contoso, Ltd. to appear as if they were sent from Wingtip Toys. All outbound messages that relate to Wingtip Toys are sent through their Edge Transport servers, and all contoso.com email addresses are rewritten to wingtiptoys.com email addresses.

    • Inbound messages for support@wingtiptoys.com are accepted by Wingtip Toy's Edge Transport servers, rewritten, and then routed to the support@contoso.com email address.

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A standard SMTP email message consists of a message envelope and message content. The message envelope contains information required for transmitting and delivering the message between SMTP mail servers. The message content contains message header fields (collectively called the message header) and the message body. The message envelope is described in RFC 2821, and the message header is described in RFC 2822.

When a sender composes an email message and submits it for delivery, the message contains the basic information required to comply with SMTP standards, such as a sender, a recipient, the date and time that the message was composed, an optional subject line, and an optional message body. This information is contained in the message itself and, by definition, in the message header.

The sender's mail server generates a message envelope for the message by using the sender’s and recipient’s information found in the message header. It then transmits the message to the Internet for delivery to the recipient's mail server. Recipients never see the message envelope because it's generated by the message transmission process, and it isn't actually part of the message.

Address rewriting changes an email address by rewriting specific fields in the message header or message envelope. Address rewriting changes several fields in outbound messages but only one field in inbound email messages. The following table shows which SMTP header fields are rewritten in outbound and inbound messages.

Message fields rewritten on outbound and inbound messages

Field name Location Outbound messages Inbound messages

MAIL FROM

Message envelope

Rewritten

Not rewritten

RCPT TO

Message envelope

Not rewritten

Rewritten

To

Message header

Rewritten

Not rewritten

Cc

Message header

Rewritten

Not rewritten

From

Message header

Rewritten

Not rewritten

Sender

Message header

Rewritten

Not rewritten

Reply-To

Message header

Rewritten

Not rewritten

Return-Receipt-To

Message header

Rewritten

Not rewritten

Disposition-Notification-To

Message header

Rewritten

Not rewritten

Resent-From

Message header

Rewritten

Not rewritten

Resent-Sender

Message header

Rewritten

Not rewritten

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Address rewriting doesn't modify any message header fields that would break SMTP functionality. For example, modifying certain header fields can affect routing loop detection, invalidate the signature, or make a rights-protected message unreadable. Therefore, the following header fields aren't modified by address rewriting.

  • Return-Path

  • Received

  • Message-ID

  • X-MS-TNEF-Correlator

  • Content-Type Boundary=string

  • Header fields located inside MIME body parts

Address rewriting ignores domains that aren't controlled by the Exchange organization. In other words, address rewriting doesn't rewrite header fields that contain domains for which the Exchange organization isn't authoritative. Rewriting such domains would cause an uncontrollable form of message relay.

Address rewriting also doesn't modify the header fields of messages that are embedded in another message. Senders and recipients expect embedded messages to remain intact and be delivered without modification, as long as the messages don't trigger transport rules that are implemented between the sender and recipient.

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Outbound-only address rewriting on an Edge Transport server modifies the sender's email address as the message leaves the Exchange organization. You can configure outbound-only address rewriting for a single user (chris@contoso.com to support@contoso.com) or for all users in a single domain (contoso.com to fabrikam.com). You are required to configure outbound-only address rewriting for users in multiple subdomains (*.fabrikam.com to .contoso.com).

The rewritten email address must be configured as a proxy address on the affected recipients. For example, if laura@sales.contoso.com is rewritten to laura@contoso.com, the proxy address laura@contoso.com must be configured on Laura's mailbox. This allows replies and inbound messages to be delivered correctly.

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Inbound and outbound, or bidirectional address rewriting on an Edge Transport server modifies the sender's email address in messages that leave the Exchange organization, and it modifies the recipient's email address in messages that enter the Exchange organization.

You can configure outbound-only address rewriting for a single user (chris@contoso.com to support@contoso.com) and all users in a single domain (contoso.com to fabrikam.com). You can’t configure bidirectional address rewriting for users in multiple subdomains (*.fabrikam.com to contoso.com).

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When you flatten multiple internal domains or subdomains into a single external domain, you need to consider the following factors:

  • Verify unique aliases   All email aliases (the part to the left of the @ sign) must be unique across all subdomains. For example, if there is a joe@sales.contoso.com, there can't be a joe@marketing.contoso.com because the rewritten email address for both users would be joe@contoso.com.

  • Add proxy addresses   The rewritten email address must be configured as a proxy address for all affected senders in the affected domains. For example, if joe@sales.contoso.com is rewritten to joe@contoso.com, you need to add the proxy address joe@contoso.com to Joe's mailbox. This allows replies and inbound messages to be delivered correctly.

  • Mail contacts for non-Exchange organizations   If you're rewriting email addresses from a non-Exchange email system, you need to create email contacts in Exchange to represent the users in the non-Exchange email system. These email contacts must contain the original email addresses and the rewritten email addresses. For example, if joe@unix.contoso.com is rewritten to joe@contoso.com, you need to create a mail contact with joe@unix.contoso.com as the external email address and joe@contoso.com as a proxy address.

When you rewrite email addresses in multiple subdomains, you need to make sure that all email aliases are unique across all your subdomains. 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

  • adam@research.contoso.com

Suppose you want to rewrite the subdomains sales.contoso.com, marketing.contoso.com, and research.contoso.com into the single domain contoso.com.

When the email addresses in each subdomain are rewritten, a conflict occurs between chris@sales.contoso.com and chris@research.contoso.com because both email addresses are rewritten to chris@contoso.com. To resolve this situation, you need to change the email address of one of the affected recipients. For example, you can change chris@research.contoso.com to christopher@research.contoso.com so the email address is rewritten to christopher@contoso.com.

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If a user's email address matches multiple address rewrite entries, the email address is only rewritten once based on the closest match. The following list describes the order of precedence of address rewrite entries from highest priority to lowest priority:

  1. Individual email addresses   An address rewrite entry is configured to rewrite the email address of john@contoso.com to support@contoso.com.

  2. Domain or subdomain mapping   An address rewrite entry is configured to rewrite all contoso.com email addresses to northwindtraders.com or all sales.contoso.com email addresses to contoso.com.

  3. Domain flattening   An address rewrite entry is configured to rewrite *.contoso.com email addresses to contoso.com.

For example, consider an Edge Transport server where the following outbound address rewrite entries are configured:

  • *.contoso.com email addresses are rewritten to contoso.com

  • japan.sales.contoso.com email addresses are rewritten to contoso.jp

If masato@japan.sales.contoso.com sends an email message, the address is rewritten to masato@contoso.jp, because that entry most closely matches the sender's email address.

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Address rewriting shouldn't affect most signed, encrypted, or rights-protected messages. If address rewriting were to invalidate or otherwise change the security status of these types of messages in any way, address rewriting isn't applied.

The following values can be rewritten because the information isn't part of message signing, encryption, or rights protection:

  • Fields in the message envelope

  • Top-level message body headers

The following values aren't rewritten because the information is part of message signing, encryption, or rights protection:

  • Header fields located inside MIME body parts that may be signed

  • The boundary string parameter of the MIME content type

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