Once you add a domain, before creating mailboxes under the domain, you will need to verify the ownership that the domain belongs to you and activate the domain.

To verify the domain, you need to add a few DNS records on to your domain's DNS panel

Visit the Domain DNS panel
Add the relevant records mentioned on the domain verification page.
Once they are added, allow few hours to propagate
Click the green Verify button to start the verification process

Please note that it might take up to 24 hours for the DNS records to propagate.
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