How can I set the DNS for example for: mail.contoso.com (or if it is possible for contoso.com for it would be the best case escenario) to be available in my personal Microsoft 365 Outlook Premium domain.
Do you mean that you want to use "contoso.com for your Office 365 mailbox? Such as, change your Office 365 mailbox from user@keyman .onmicrosoft.com to user@Company portal .com?
If so, you need have Microsoft 365 admin center license and Exchange online license first. After that, you need to have the ownership of this domain, then you will could add your domain to Office 365, then you can use this domain name as your email address suffix.
Please correct me if I am wrong.
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