How to List All Accepted Domains in Microsoft 365

The straightforward answer is ‘No’. A Microsoft 365 organization can function without its own domain. This section explains how a Microsoft 365 tenant operates without your own custom domains. It’s also important to understand the concepts listed here, as the individual domains added to the Microsoft 365 environment are considered accepted domains in Exchange Online.

Yes, when you register for Microsoft 365, your organization is assigned a default domain like ‘company.onmicrosoft.com’. This domain is used by default for email addresses or aliases unless you add a custom domain and set it as the default for your Microsoft 365 setup.

You can also add another onmicrosoft.com domain if needed. A common reason for adding additional onmicrosoft.com domains is to perform a SharePoint domain rename to match the organization’s rebranding. Once an onmicrosoft.com domain is added, it cannot be removed from the Microsoft 365 environment.

By default, the onmicrosoft.com domain provided when creating the Microsoft 365 tenant will be the fallback domain. However, you can change the fallback domain by adding another onmicrosoft.com domain later.

A fallback domain in Microsoft 365 is used as a backup when the primary domain cannot be used for certain operations or configurations. A fallback domain is particularly useful when the primary domain is unavailable, ensuring that your users' emails are still routed successfully.

You can add up to 5,000 domains to a Microsoft 365 subscription, including both custom domains and a maximum of 5 onmicrosoft.com domains.

Two key configurations to manage and route the emails in Exchange Online are accepted domains and remote domains. While both are essential for configuring your email system, they serve different purposes. Refer to the sections below for a detailed comparison of accepted domain vs remote domain in Microsoft 365.

Accepted domains are the custom domains you add to your Microsoft 365 tenant to allow mail flow. In simple terms, these are the domains used to define where email is accepted and delivered for your organization.

Typically, organizations configure various domains based on departments, branches, or sections, and create mailbox accounts to receive or send emails.

Example: If you have an accepted domain named ‘company.com’, emails sent to ‘person1@company.com’ or ‘person2@company.com’ are delivered to your organization’s mailboxes. This is true if the recipients exist within your organization.

Remote domains are external domains that define how emails are formatted and handled when sending to or receiving from external mail systems. They are used to manage features such as email format, automatic replies, and non-delivery reports for emails sent to the external domains.

By default, a remote domain with a set of rules is defined to send and receive messages from external organizations. Any changes made to the default remote domain will apply to all external domains. To apply settings to a specific external domain, you need to create a dedicated remote domain entry.

Example: If your organization regularly interacts with another organization that uses the domain ‘company1.com,’ you can create a remote domain entry for ‘company1.com.’ This configuration allows you to specify how emails sent to ‘company1.com’ are managed.

Adding a domain to Microsoft 365 is essential for organized email management. By default, when you add a domain in Microsoft 365, it becomes part of your organization’s accepted domains list in Exchange Online.

Adding your domain to the Microsoft 365 services requires the following navigation.

Once you add a domain to the Microsoft 365 environment, it will be automatically recognized as an accepted domain of the type 'authoritative.' However, you can change the accepted domain type from authoritative to internal relay. Refer the points below for a comparison of authoritative vs internal relay in Microsoft 365:

• To modify the accepted domain type, select the desired mail domain from the ‘Accepted domains’ page in the Exchange admin center.
• Select the appropriate domain type and click Save to apply changes.

Alternatively, you can use the ‘Set-AcceptedDomain’ cmdlet in the Exchange Online PowerShell to update the accepted domain type.

Set-AcceptedDomain -Identity <DomainName> -DomainType <Authoritative/InternalRelay>

Make sure to replace ‘<DomainName>’ with the desired domain name before execution.

• The accepted domain configuration report in AdminDroid shows all changes made to accepted domains in Exchange Online.
• To closely track any unauthorized activity, it provides metrics such as the operation performed, who carried it out, the domain affected, and more.

Tip: Make use of the Alert (🔔) option available with this audit report to instantly get notified when a configuration change is carried out in any accepted domains.

Admins may choose to block a domain from sending emails in Microsoft 365 for several reasons, including:

• Go to the ‘Accepted domains’ page in the Exchange admin center and click on the appropriate domain.
• Disable the Allow mail to be sent from this domain check box and click Save.

You can also use the following PowerShell cmdlet in the Exchange Online module to prevent sending emails from specific domains.

Set-AcceptedDomain -Identity <DomainName> -SendingFromDomainDisabled $true

Similarly, you can block domains from receiving emails, especially for domains that are intended for one-way communication (e.g., @noreply.company.com) where replies are not required. To block such domains, use the following command.

Set-AcceptedDomain -Identity <DomainName> -SendingToDomainDisabled $true 

Note: To block incoming emails for a specific domain, you must use PowerShell as the Exchange admin center does not support this action.

Organizations often use subdomains to distinguish different departments or project teams (e.g., sales.company.com, support.company.com). In such cases, adding these sub domains as accepted domains in Exchange Online is essential to properly route and deliver the mails within the organization.

But what if you have a hybrid environment where some subdomains exist only on-premises? In this scenario, you need to enable email flow for these subdomains using the following steps.

Note: If you have a few subdomains, it’s recommended to configure them as accepted domains rather than subdomains.

The initial step in enabling mail flow to on-premises subdomains is to set up matching subdomains for the parent domain. Follow these steps to complete the process:

• Navigate to Mail Flow»Accepted domains»Appropriate domain in the Exchange admin center.
• Ensure the ‘Internal relay’ option is selected and enable the Accept mail for all subdomains check box. Then, click Save.

To enable matching subdomains in an accepted domain using PowerShell, execute the following command. Ensure the domain type is set to ‘Internalrelay’ and that you have entered the desired domain name before execution.

Set-AcceptedDomain -Identity <DomainName> -MatchSubdomains $true

After enabling the domain to accept mail for its subdomains, you must configure it to transmit messages from Office 365 to your organization's email server. Here’s how to add the domain to the connector for message transmission.