Multiple O365 domains

Gavin Ross 231 Reputation points
2021-10-18T20:41:55.797+00:00

I have a question regarding setting up domains in O365. We currently have a Hybrid setup, no mailboxes are in O365, only on-prem, all mail flows through our SPAM filter on-prem and MX still points to our Firewall. We were just notified that we needed to create a new domain and we were thinking that it would be easier to have all the mailboxes in O365. The question I have is can we have 1 domain only use O365 for mail routing and keep the other domain on-prem. We would also need to archive to a separate archiving service only from this new domain. Not sure how to go about this, so any advice would be great.

Thanks,
Gavin

Microsoft Exchange Online Management
Microsoft Exchange Online Management
Microsoft Exchange Online: A Microsoft email and calendaring hosted service.Management: The act or process of organizing, handling, directing or controlling something.
4,166 questions
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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Microsoft Exchange Hybrid Management
Microsoft Exchange Hybrid Management
Microsoft Exchange: Microsoft messaging and collaboration software.Hybrid Management: Organizing, handling, directing or controlling hybrid deployments.
1,875 questions

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  1. Andy David - MVP 141K Reputation points MVP
    2021-10-18T20:43:55.417+00:00

    All the domains you want to route messages through 365 must exist in 365, so you would need to add and verify all that are needed. - even if just routing to on-prem.

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  2. Joyce Shen - MSFT 16,641 Reputation points
    2021-10-19T03:04:44.947+00:00

    Hi @Gavin Ross

    Agree with Andy, and below are some official document introduces about the Exchange hybrid mailflow, mail flow using a third-party cloud service with Exchange Online for your reference as well.

    Transport routing in Exchange hybrid deployments

    • 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.

    Also note that: 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.

    Manage mail flow using a third-party cloud service with Exchange Online

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  3. Gavin Ross 231 Reputation points
    2021-10-22T21:45:55.48+00:00

    Thanks, probably just going to go with a new tenant.

    Thanks,
    Gavin