📖 What is Custom Domain Name?
A Custom Domain Name is a domain purchased from a registrar and verified in Microsoft 365 to allow users to send and receive emails using a professional address. This replaces the default .onmicrosoft.com domain provided during tenant creation.
"You must prove ownership via DNS records (typically TXT or MX records) before the domain can be successfully added to the tenant."
📚 Certification: Microsoft 365 Administrator (MS-102)
🔑 What are the Key Concepts of Custom Domain Name?
- ▸ DNS Verification requires adding a TXT or MX record to the domain's DNS settings to prove ownership before Microsoft 365 enables the domain.
- ▸ The Primary Domain is the default address used for new users and services, though multiple custom domains can be added as aliases.
- ▸ Updating the User Principal Name (UPN) is necessary for users to sign in using their professional custom domain instead of the default tenant address.
- ▸ Configuring MX, CNAME, and TXT records ensures proper mail routing, service discovery, and email authentication via SPF, DKIM, and DMARC protocols.
🎯 How does Custom Domain Name appear on the MS-102 Exam?
You may be asked to identify the correct sequence of steps for rebranding a tenant, starting with DNS verification and ending with updating user UPNs and email addresses to the new domain.
A scenario might describe a situation where a company has successfully verified a domain but cannot receive mail; you must identify that the MX record is missing or incorrectly configured.
Expect questions about managing multiple custom domains during a corporate merger, where you must decide between assigning a new primary domain or using alias domains for the acquired company.
❓ Frequently Asked Questions
Can the default .onmicrosoft.com domain be completely removed?
No, the initial .onmicrosoft.com domain is a permanent part of the tenant. It serves as a fallback for routing and is required for certain administrative functions and internal system identifiers.
What is the difference between a primary domain and an alias domain?
The primary domain is the default for new accounts and the organization's main identity. Alias domains allow users to receive mail at different addresses without creating separate mailboxes.
Why is DNS verification required before configuring mail flow?
Verification prevents unauthorized users from claiming domains they don't own. By requiring a TXT or MX record, Microsoft ensures the administrator has actual control over the domain's DNS settings.