Outbound email getting routed to office 365 from on premise mailbox


  • Hi Team,

    Asking for your expert opinion.

    Quick run down of the environment

    Exchange 2010 recently configured for hybrid co existence

    3rd Party email security appliance

    Before hybrid normal outbound flow is on prem -> 3rd party -> internet ( recipients MX record )

    After configuring Hybrid, whenever a user emails an external recipient that have office 365 mx record the email is getting routed to intenal office 365 before going to the internet.

    on prem -> 3rd party -> office 365 -> internet ( recipients mx record )

    This is not happening on recipient that does not have office 365 for their mx record ( gmail, mimecast, on prem other email host etc )

    No changes has been made to the 3rd party appliance.

    Thanks in advance

    Donnerstag, 17. Mai 2018 16:14

Alle Antworten

  • Hi Jake,

    I think the outbound flow works as expected. For your on-premise exchange environment, Office 365 itself is an outside organization. When an on-premise mailbox sends an email to an external recipient which is located on Office 365, in your scenario, it should be as below:

    on prem -->3rd party --> Office 365 

    You can refer to the following article to understand hybrid mail flow in different scenarios in detail :

    Mail flow best practices for Exchange Online and Office 365 (overview)

    Best Regards,
    Niko Cheng

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    Freitag, 18. Mai 2018 07:39
  • Thank you for the response Niko.

    That what I originally thought but I can see those emails from our office 365 message trace. To elaborate more the flow goes like this

    on prem -> 3rd party -> our office 365 -> recipients office 365.

    I cannot find the header but if I remember correctly there's an obe.outbound on one of the transit which I only see whenever an email comes from office 365 or get routed to o365 ( EOP plans ).

    Freitag, 18. Mai 2018 15:28
  • Are you using Centralized Mail Transport(CMT) for Hybrid, If CMT is enabled for Hybrid, all mails to external recipients for O365 users passes on-prem. You can run the cmdlet, Get-hybridconfiguration, under features you see centralized transport if enabled.




    Disclaimer: The views expressed on this blog are my own and do not necessarily reflect the views of my employer.

    Freitag, 18. Mai 2018 17:55