Fix Synced Microsoft 365 User Email Reverting to onmicrosoft.com
Question details
A synchronized Microsoft 365 user's primary email address unexpectedly changes to the tenant's default onmicrosoft.com domain.
- Product
- Microsoft 365
- Device & OS
- not provided
- Scenario
- Managing user accounts and email addresses in a hybrid Microsoft 365 and on-premises Active Directory environment.
- Observed behavior
- The user's primary email address reverts to the default tenant onmicrosoft.com domain instead of keeping the assigned custom domain as the default.
Ensure you have administrative privileges to both your on-premises Active Directory and the Microsoft 365 admin center before making attribute changes.
Check and Update the Active Directory proxyAddresses Attribute
Ensure the desired custom domain email is set as the primary SMTP address using uppercase formatting.
In a hybrid environment, Microsoft 365 relies on on-premises Active Directory attributes to dictate email addresses. If the primary email is not explicitly defined in the proxyAddresses attribute, Azure AD Connect may default to the onmicrosoft.com routing domain.
Log in to your on-premises server and open 'Active Directory Users and Computers'.
Click on the 'View' menu at the top and select 'Advanced Features' to reveal additional tabs.
Locate the affected user account, right-click on it, and select 'Properties'.
Navigate to the 'Attribute Editor' tab, scroll down to find the 'proxyAddresses' attribute, and click 'Edit'.
Ensure that the intended primary email address is prefixed with an uppercase 'SMTP:' (e.g., SMTP:user@customdomain.com). Lowercase 'smtp:' is strictly for secondary aliases.
Apply the changes and force an Azure AD Connect synchronization using PowerShell or wait for the standard sync cycle.
Verify User Principal Name (UPN) Suffix Configuration
Confirm the user's UPN suffix matches the verified custom domain to prevent fallback to the default tenant domain.
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Frequently Asked Questions
Why does Microsoft 365 keep changing my custom domain to onmicrosoft.com?
This commonly occurs in a hybrid setup when the on-premises Active Directory doesn't explicitly define the primary SMTP address using the correct capitalization in the proxyAddresses attribute, causing Azure AD Connect to fall back to the tenant's default routing domain.
What is the difference between uppercase SMTP and lowercase smtp in Active Directory?
In the proxyAddresses attribute, an uppercase 'SMTP:' designates the primary, default email address for the user, while a lowercase 'smtp:' is used for adding secondary email aliases that the user can receive mail from.
How long does it take for on-premises AD changes to sync to Microsoft 365?
By default, Azure AD Connect synchronizes directory changes every 30 minutes. You can also manually force a delta sync using the ADSync PowerShell module to apply your changes immediately.
