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EmailAdministrator-6879 asked LucasLiu-MSFT commented

Default domain clarification

Migrating from standalone mail server (Icewarp) to EOL. Have created and verified my domain (topperfloats.com) in EOL. MS domain (toppermail.onmicrosoft.com) is set as default domain and both are authoritative. The two mail servers will not be connected/related in any way.

All users use Thunderbird and I've successfully reconfigured my Thunderbird-Icewarp account to use the Icewarp server IP and created a TB user account to "toppermail.onmicrosoft.com" and have sent mail to/from gmail via the EOL-Thunderbird account. Have created my user accounts in the EOL topperfloats.com domain.

From a DNS perspective, what is the effect of making topperfloats.com the default? I'm trying to grok as many issues as I can before doing the cutover (<25 users) as EOL limits my ability to track down missing emails (compared to Icewarp). FWIW, no changes have been made to DNS yet. Thanks, Tom

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Hi @EmailAdministrator-6879 ,
Do suggestions above help? If the issue has been resolved, please click “Accept as answer” to mark helpful reply as an answer, this will make answer searching in the forum easier and be beneficial to other community members as well.

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Hi @EmailAdministrator-6879 ,
I am writing here to confirm with you how thing going now? If the above suggestion helps, please click “Accept as answer” to mark helpful reply as an answer.Your action would be helpful to other users who encounter the same issue and read this thread.
Thanks for your understanding.



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AndyDavid answered LucasLiu-MSFT commented

You mean making it the default reply address for users?

If so,

Ensure you have DMARC and SPF and the MX records correctly set this domain to point to Office 365:

https://docs.microsoft.com/en-us/microsoft-365/security/office-365-security/use-dmarc-to-validate-email?view=o365-worldwide

Also be sure to enable DKIM for this domain as well (topperfloats.com)

https://docs.microsoft.com/en-us/microsoft-365/security/office-365-security/use-dmarc-to-validate-email?view=o365-worldwide#step-3-set-up-dkim-for-your-custom-domain

Once you do that, you will be good to go from a mail flow perspective.

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Hi @EmailAdministrator-6879 ,
Agree with Andy.
SPF, DKIM and DMARC will protect your organization from receiving spam email. MX ensures that your organization can successfully receive mail. After you finish editing, please test the mail flow. And please make sure that the correct CNAME record is configured.



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EmailAdministrator-6879 answered LucasLiu-MSFT edited

Not about default reply address.

Question is about the ramifications of changing my domain (topperfloats.com) to the default in EOP. What happens under the hood?

Using this to setup DNS records:

https://docs.microsoft.com/en-us/microsoft-365/admin/dns/create-dns-records-at-dnsmadeeasy?view=o365-worldwide

Have not made the DNS changes yet as I want to make sure I understand how it all is going to work upon cutover. I don't like tracking down problems I didn't know were going to be problems. Hence, my question.

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Where exactly are you changing this?

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Hi @EmailAdministrator-6879 ,
If no changes are made to your MX records, the email sent to "topperfloats.com" will still be successfully sent to the user's mailbox.
In addtion, you also need to set the correct DMARC, SPF and DKIM as mentioned above.



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