Exchange Connection Problems

How to Set Up Email Addresses for a Microsoft 365 Subdomain

Algirdas Jasaitis Oct 10, 2026 869 views

Question details

The user needs to configure email addresses for a specific subdomain (such as for sales or marketing) in Microsoft 365 and enable proper mail flow.

How to Set Up Email Addresses for a Microsoft 365 Subdomain

Product: Microsoft 365
Device & OS: not provided
Scenario: Setting up department-specific email addresses on a subdomain to handle outbound and inbound communication.
Observed behavior: Successfully configuring the subdomain as an accepted domain, establishing working mail flow, and understanding license requirements for outbound emails.

Before you start

Ensure you have global administrator or Exchange administrator privileges in your Microsoft 365 admin center, and that the parent domain is already successfully verified.

Solution 1Recommended

Configure the Subdomain as an Accepted Domain and Enable Mail Flow

Step-by-step instructions to add a subdomain and set it up for email flow in Exchange Online.

Subdomains are usually verified automatically if their parent domain is already verified in Microsoft 365. To send and receive emails from this subdomain, you must configure it as an accepted domain and ensure users have the appropriate licenses.

Verify Domain Status

Log in to the Microsoft 365 Admin Center, navigate to Settings > Domains, and confirm that the parent domain is verified. The subdomain inherits this verification automatically.

Access Exchange Admin Center

Open the Exchange admin center (EAC) from the admin center dashboard.

Configure Accepted Domain

Go to Mail flow > Accepted domains. Add the subdomain and configure it as an authoritative or internal relay domain depending on your organization's routing needs.

Assign Licenses

Navigate to Users > Active users. Ensure you assign an active Microsoft 365 license with Exchange Online to each user or shared mailbox that will send emails from this new subdomain.

Configure the Subdomain as an Accepted Domain and Enable Mail Flow

Licensing Requirement: Every user actively sending outbound email through the configured subdomain requires a valid Microsoft 365 license that includes Exchange Online. Shared mailboxes generally do not require a license unless they exceed 50 GB or require direct sign-in.

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Frequently Asked Questions

Do I need to manually verify a subdomain in Microsoft 365?

No. If the parent domain is already verified in your Microsoft 365 tenant, any subdomain you add is usually verified automatically.

Why is outbound email not working for my newly added subdomain?

Outbound email failures often occur if the sender's mailbox lacks a proper Microsoft 365 license, or if the subdomain hasn't been properly configured as an accepted domain in the Exchange admin center.

Can I configure my subdomain as an Internet relay?

Yes. You can configure the subdomain as an internal relay in the Exchange admin center if you need to route messages to an on-premises email server or a third-party service.

Do I need a new connector for my subdomain?

You only need a new connector if you are routing mail for that subdomain to a third-party service, an on-premises Exchange server, or an external Internet relay.

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