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LoremanReturns9791's avatar
LoremanReturns9791
Copper Contributor
Sep 05, 2023

External DNS records required for email in Office 365 (Exchange Online)

Hi all, we have a configuration with an OnPrem Exchange Server 2019 Infra in hybrid with Exchange Online. Looking at the MS documentation avaliable at this link: https://learn.microsoft.com/en-us/m...
exchange online
Exchange Server
hybrid
Dan_Snape's avatar
Dan_Snape
Bronze Contributor
Sep 06, 2023
Elie is 100% correct. The documentation that you refer to is related to external DNS and in a scenario where all mailboxes are located in Exchange Online and none are located in on-premises Exchange. If that is your scenario, then create a CNAME record for autodsicover.<your domain> that points to autodiscover.outlook.com.
If you have mailboxes located on-premises (even if it's just a few) then you need to keep the external DNS record for autodiscover pointing to your on premises Exchange server. You will need to keep this configuration in place until all mailboxes are migrated to Exchange Online (or at least all those mailboxes an Outlook client connects to)
LoremanReturns9791's avatar
LoremanReturns9791
Copper Contributor
Sep 07, 2023

Hi Dan_Snape thanks for your reply.

I managed (and actually manage) a lot of hybrid configurations. No doubt on how autodiscover must be configured based on the scenario.

My question is related on the autodiscover.service.contoso.com CNAME record only.

Please read!!! I'm not referring to autodiscover.contoso.com, i'm referring to autodiscover.service.contoso.com

If i've OnPrem mailboxes in a hybrid scenario, do you mean i've to create a CNAME record for autodiscover.service.contoso.com that point to the Exchange OnPrem web services?

I never create this record, everything works fine, the record is suggested based on the MS documentation. What's the purpose of autodiscover.service.contoso.com record? Which is the difference from autodiscover.contoso.com?

 

Thanks!

  • Dan_Snape's avatar
    Dan_Snape
    Bronze Contributor
    Sep 10, 2023
    It's not mentioned in the M365 admin portal when you you look at the required DNS records for a custom domain. Maybe Microsoft can provide more insights...perhaps it's for a future product/configuration
  • LoremanReturns9791's avatar
    LoremanReturns9791
    Copper Contributor
    Sep 08, 2023

    Hi Dan_Snape, thanks for your reply.

    I also thoungh it's just an error.

    But this CNAME record is mentioned in two Microsoft documentation pages.

    Here: https://learn.microsoft.com/en-us/microsoft-365/enterprise/external-domain-name-system-records?view=o365-worldwide#external-dns-records-required-for-email-in-office-365-exchange-online

    and

    here: https://learn.microsoft.com/en-us/training/modules/configure-client-connectivity-microsoft-365/3-explore-dns-records-required-client-configuration

    In the second one it refers to the CNAME autodiscover.service.adatum.com for Exchange Federation as "needed when you deploy Exchange in a hybrid configuration with Microsoft 365"

    That's a news for me, never configured "autodiscover.service.adatum.com" in any Hybrid Configuration and federation works fine.

  • Dan_Snape's avatar
    Dan_Snape
    Bronze Contributor
    Sep 07, 2023
    No you don't. I believe that's just an error in the article. You should use the "Submit feedback" option to get it fixed (Microsoft will update it very quickly)