Default domain clarification

Email Administrator 1 Reputation point

Migrating from standalone mail server (Icewarp) to EOL. Have created and verified my domain ( in EOL. MS domain ( 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 "" and have sent mail to/from gmail via the EOL-Thunderbird account. Have created my user accounts in the EOL domain.

From a DNS perspective, what is the effect of making 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

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.
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  1. Andy David - MVP 145.1K Reputation points MVP

    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:

    Also be sure to enable DKIM for this domain as well (

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

  2. Email Administrator 1 Reputation point

    Not about default reply address.

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

    Using this to setup DNS records:

    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.