Transport routing in Exchange hybrid deployments

  • Article

This topic discusses your routing options for inbound messages from the Internet and outbound messages to the Internet.

Important

Don't place any servers, services, or devices between your on-premises Exchange servers and Microsoft 365 or Office 365 that process or modify SMTP traffic. Secure mail flow between your on-premises Exchange organization and Microsoft 365 or Office 365 depends on information contained in messages sent between the organization. Firewalls that allow SMTP traffic on TCP port 25 through without modification are supported. If a server, service, or device processes a message sent between your on-premises Exchange organization and Microsoft 365 or Office 365, this information is removed. If this happens, the message will no longer be considered internal to your organization and will be subject to anti-spam filtering, transport and journal rules, and other policies that may not apply to it.

Note

The examples in this topic don't include the addition of Edge Transport servers into the hybrid deployment. The routes messages take between the on-premises organization, the Exchange Online organization, and the Internet don't change with the addition of an Edge Transport server. The routing only changes within the on-premises organization. For more information about adding Edge Transport servers to a hybrid deployment, see Edge Transport servers with hybrid deployments.

Inbound messages from the Internet

As part of planning and configuring your hybrid deployment, you need to decide whether you want all messages from Internet senders to be routed through Exchange Online or your on-premises organization. All messages from Internet senders will initially be delivered to the organization you select and then routed according to where the recipient's mailbox is located. Whether you choose to have messages routed through Exchange Online or your on-premises organization depends on various factors, including whether you want to apply compliance policies to all messages sent to both organizations, how many mailboxes are in each organization, and so on.

The path messages sent to recipients in your on-premises and Exchange Online organizations take depends on how you decide to configure your MX record in your hybrid deployment. The preferred method is to configure your MX record to point to Exchange Online Protection (EOP) in Microsoft 365 and Office 365 as this configuration provides the most accurate spam filtering. The Hybrid Configuration wizard doesn't configure the routing for inbound Internet messages for either the on-premises or Exchange Online organizations. You must manually configure your MX record if you want to change how your inbound Internet mail is delivered.

  • If you change your MX record to point to the Exchange Online Protection service in Microsoft 365 or Office 365: This is the recommended configuration for hybrid deployments. All messages sent to any recipient in either organization will be routed through the Exchange Online organization first. A message addressed to a recipient that's located in your on-premises organization will be routed first through your Exchange Online organization and then delivered to the recipient in your on-premises organization. This route is recommended if you have more recipients in your Exchange Online organization than in your on-premises organization. This configuration option is required for Exchange Online Protection to provide scanning and blocking for spam.

  • If you decide to keep your MX record pointed to your on-premises organization: All messages sent to any recipient in either organization will be routed through your on-premises organization first. A message addressed to a recipient that's located in Exchange Online will be routed first through your on-premises organization and then delivered to the recipient in Exchange Online. This route can be helpful for organizations where you have compliance policies that require messages sent to and from an organization be examined by a journaling solution. If you pick this option, Exchange Online Protection will not be able to effectively scan for spam messages.

For more information, see Mail flow best practices for Exchange Online, Microsoft 365, and Office 365 (Overview).

Read the section below that matches how you plan to route messages sent from Internet recipients to your on-premises and Exchange Online recipients. In each section, the "on-premises Exchange server" can be either an Exchange 2013 Client Access server or an Exchange 2016 mailbox server.

Route incoming Internet messages through the Exchange Online organization

The following steps and diagrams illustrate the inbound message path that occur in your hybrid deployment if you decide to point your MX record to the EOP service in the Microsoft 365 or Office 365 organization. The message path differs depending on whether you choose to enable centralized mail transport.

Important

You may need to purchase EOP licenses for each on-premises mailbox that receives messages that are first delivered to EOP and then routed through the Exchange Online organization. Contact your Microsoft reseller for more information.

When centralized mail transport is disabled (default configuration), incoming Internet messages are routed as follows in a hybrid deployment:

  1. An inbound message is sent from an Internet sender to the recipients julie@contoso.com and david@contoso.com. Julie's mailbox is located on an Exchange Mailbox server in the on-premises organization. David's mailbox is located in Exchange Online.

  2. Because the recipients both have contoso.com email addresses, and the MX record for contoso.com points to EOP, the message is delivered to EOP.

  3. EOP routes the messages for both recipients to Exchange Online.

  4. Exchange Online scans the messages for viruses and performs a lookup for each recipient. Through the lookup, it determines that Julie's mailbox is located in the on-premises organization while David's mailbox is located in the Exchange Online organization.

  5. Exchange Online splits the message into two copies. One copy of the message is delivered to David's mailbox.

  6. The second copy is sent from Exchange Online back to EOP.

  7. EOP sends the message to an on-premises Exchange server in the on-premises organization.

  8. An Exchange server sends the message to the Exchange Mailbox server where it's delivered to Julie's mailbox.

    Route mail through the Exchange Online organization for both on-premises and Exchange Online organizations with centralized mail transport disabled (default configuration)

    Screenshot that shows Centralized inbound to EXO.

When centralized mail transport is enabled, incoming Internet messages are routed as follows in a hybrid deployment:

  1. An inbound message is sent from an Internet sender to the recipients julie@contoso.com and david@contoso.com. Julie's mailbox is located on an Exchange Mailbox server in the on-premises organization. David's mailbox is located in Exchange Online.

  2. Because the recipients both have contoso.com email addresses, and the MX record for contoso.com points to EOP, the message is delivered to EOP and scanned for viruses.

  3. Since centralized mail transport is enabled, EOP routes the messages for both recipients to an on-premises Exchange server.

  4. The on-premises Exchange server performs a lookup for each recipient. Through the lookup, it determines that Julie's mailbox is located in the on-premises organization while David's mailbox is located in the Exchange Online organization.

  5. The on-premises Exchange server splits the message into two copies. One copy of the message is delivered to Julie's mailbox on the on-premises Exchange Mailbox server.

  6. The second copy is sent from the on-premises Exchange server back to EOP.

  7. EOP sends the message to Exchange Online.

  8. Exchange Online delivers the message to David's mailbox.

    Route mail through the Exchange Online organization for both on-premises and Exchange Online organizations with centralized mail transport enabled

    Screenshot that shows Inbound EXO, centralized enabled.

Route incoming Internet messages through your on-premises organization

The following steps and diagram illustrate the inbound Internet message path that will occur in your hybrid deployment if you decide to keep your MX record pointed to your on-premises organization.

  1. An inbound message is sent from an Internet sender to the recipients julie@contoso.com and david@contoso.com. Julie's mailbox is located on an Exchange Mailbox server in the on-premises organization. David's mailbox is located in Exchange Online.

  2. Because the recipients both have contoso.com email addresses, and the MX record for contoso.com points to the on-premises organization, the message is delivered to an on-premises Exchange server.

  3. The on-premises Exchange server performs a lookup for each recipient using an on-premises global catalog server. Through the global catalog lookup, it determines that Julie's mailbox is located on an on-premises Exchange Mailbox server while David's mailbox is located in the Exchange Online organization and has a hybrid routing address of david@contoso.mail.onmicrosoft.com.

  4. The on-premises Exchange server splits the message into two copies. One copy of the message is sent to the on-premises Exchange Mailbox server where it's delivered to Julie's mailbox.

  5. The second copy of the message is sent by the on-premises Exchange server to EOP, which receives messages sent to the Exchange Online organization, using a Send connector configured to use TLS.

  6. EOP sends the message to the Exchange Online organization where the message is scanned for viruses and delivered to David's mailbox.

    Route mail through the on-premises organization for both on-premises and Exchange Online organizations

    Screenshot that shows Centralized inbound mail, on-premises.

Outbound messages to the Internet

In addition to choosing how inbound messages addressed to recipients to your organizations are routed, you can also choose how outbound messages sent from Exchange Online recipients are routed. When you run the Hybrid Configuration wizard, you can select one of two options:

  • Don't enable centralized mail transport: Selected by default in the Hybrid Configuration wizard, this option routes outbound messages sent from the Exchange Online organization directly to the Internet. Use this option if you don't need to apply any on-premises compliance policies or other processing rules to messages that are sent from recipients in the Exchange Online organization.

  • Enable centralized mail transport: Selecting this option routes outbound messages sent from the Exchange Online organization through your on-premises organization. Except for messages sent to other recipients in the same Exchange Online organization, all messages sent from recipients in the Exchange Online organization are sent through the on-premises organization. This enables you to apply compliance rules to these messages and any other processes or requirements that must be applied to all of your recipients, regardless of whether they're located in the Exchange Online organization or the on-premises organization.

    Note

    Centralized mail transport is only recommended for organizations with specific compliance-related transport needs. Our recommendation for typical Exchange organizations is not to enable centralized mail transport.

Messages sent from on-premises recipients are always sent to directly to Internet recipients using DNS regardless of which of the above choices you select in the Hybrid Configuration wizard.

The following steps and diagram illustrate the outbound message path for messages sent from on-premises recipients.

  1. Julie, who has a mailbox on the on-premises Exchange Mailbox server, sends a message to an external Internet recipient, erin@cpandl.com.

  2. The Exchange server looks up the MX record for cpandl.com and sends the message to the cpandl.com mail servers located on the Internet.

    Messages from on-premises senders to Internet recipients

    Screenshot that shows Outbound mail from on-premises.

Read the section below that matches how you plan to route messages sent from recipients in the Exchange Online organization to Internet recipients.

Deliver Internet-bound messages from Exchange Online using DNS (Centralized mail transport disabled)

The following steps and diagram illustrate the outbound message path for messages sent from Exchange Online recipients to an Internet recipient that occur when Enable centralized mail transport is not selected in the Hybrid Configuration wizard, which is the default configuration.

  1. David, who has a mailbox in the Exchange Online organization, sends a message to an external Internet recipient, erin@cpandl.com.

  2. Exchange Online scans the message for viruses and sends the message to the Exchange Online EOP company.

  3. EOP looks up the MX record for cpandl.com and sends the message to the cpandl.com mail servers located on the Internet.

    Mail from Exchange Online senders routed directly to the Internet with centralized mail transport disabled (default configuration)

    Screenshot that shows EXO direct outbound mail.

Route Internet-bound messages from Exchange Online through your on-premises organization (Centralized mail transport enabled)

The following steps and diagram illustrate the outbound message path for messages sent from Exchange Online recipients to an Internet recipient that occur when you select Enable centralized mail transport in the Hybrid Configuration wizard.

  1. David, who has a mailbox in the Exchange Online organization, sends a message to an external Internet recipient, erin@cpandl.com.

  2. Exchange Online scans the message for viruses and sends the message to EOP.

  3. EOP is configured to send all Internet-bound messages to an on-premises server, so the message is routed to an on-premises Exchange server. The message is sent using TLS.

  4. The on-premises Exchange server performs compliance, anti-virus, and any other processes configured by the administrator on David's message.

  5. The on-premises Exchange server looks up the MX record for cpandl.com and sends the message to the cpandl.com mail servers located on the Internet.

    Mail from Exchange Online senders routed through on-premises organization with centralized mail transport enabled

    Screenshot that shows Outbound EXO mail via on-premises.