Email Relaying from Exchange On-Prem. to O365

Rana Junaid Abrar AHMAD 21 Reputation points


I have a question regarding email relaying in Hybrid Configuration Setup as mentioned below,

Current Setup:

Exchange 2016

Exchange Online

External Email Gateway (Forcepoint Cloud Security - MX Record holder)

HCW is setup with Centralized Transport Enable.

External Email lands on FP and forwarded to Exchange on-Prem. and redirected to Exchange Online if mailbox is not available on-Prem.

New Requirement:

We want Forcepoint to Send Email to Exchange Online and from EOL to On-Prem.

Forcepoint <>EOL <> Exchange2016

As per my understanding following is the plan I have scoped.

Forcpoint: Fairly Simple

Connector to Send Emails to EOL

Connctor to Receive Emails from EOL

Exchange 2016:

Disable Send Connector (External Emails to Forcepoint)

Modify Send Connector (Emails to EOL)

change Address space from to *

Exchange Online: (O365)

New: Email Receive connector from Forcepoint IPs to Exchange Online

New: Email Send Connector Send * from Exchange Online to Forcepoint via FP Smart Host

Modify Email Send Connector From EOL to On-Prem. Exchangechange address space from * to & change -RouteAllMessagesViaOnPremises from True to False

This is high level plan. My only concern is if email from Internet to EOL, Mailbox not available on EOL, should it automatically check for send connector created from EOL to On-Prem or would I need to change Authoritative Status of my accepted domain setting?

Any other advise would also be appreciated.

Thank you & regards,

Exchange Server Management
Exchange Server Management
Exchange Server: A family of Microsoft client/server messaging and collaboration software.Management: The act or process of organizing, handling, directing or controlling something.
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Accepted answer
  1. Andy David - MVP 115.3K Reputation points MVP

    What is set for the accepted domain now in Office 365? If you are in hybrid mode and have mailboxes on-prem, then you need to set it to Internal Relay.

    You may want to also re-run the Hybrid Wizard and choose the settings you want to ensure all the correct changes are done.

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  1. Lydia Zhou - MSFT 2,361 Reputation points Microsoft Employee

    @Rana Junaid Abrar AHMAD

    In general, HCW will help to configure the connector for the mail flow between on-premises and Exchange Online. You can re-run HCW and don't enable centralized mail transport. Then check connectors settings between on-premises and Exchange Online. Here are more information for Manage mail flow using a third-party cloud service with Exchange Online and on-premises mailboxes.

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