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How to Resolve Azure AD Proxy Address Conflicts Across Multiple Domains
Question details
Users experience duplicate SMTP proxy address errors during a hybrid Exchange migration when synchronizing two on-premises domains to a single Microsoft Entra ID tenant.
- Product
- Microsoft Entra ID, Exchange Online
- Device & OS
- not provided
- Scenario
- Synchronizing multiple on-premises Active Directory domains with a single Microsoft Entra ID tenant during an Exchange hybrid migration.
- Observed behavior
- Azure AD Connect reports duplicate SMTP proxy address conflicts, even when user principal names (UPNs) are unique, because accounts share the same proxy address in the onmicrosoft.com domain.
Ensure you have Microsoft 365 Global Administrator privileges, as investigating tenant directory synchronization issues and submitting official service requests requires administrative access.
Investigate Proxy Attributes and Contact Microsoft 365 Support
Because resolving these specific proxy address conflicts requires tenant-specific directory and synchronization analysis, reviewing Microsoft documentation and opening a support request is the recommended approach.
Unique User Principal Names (UPNs) do not automatically prevent proxy address conflicts. When multiple domains sync to a single tenant, duplicate SMTP addresses can still be generated in the default onmicrosoft.com routing domain, causing Azure AD Connect to flag a conflict.
Read Microsoft's official troubleshooting documentation on 'Proxy address conflict when adding an email address in Exchange Online' to understand the common triggers for SMTP duplication.
Examine how the proxyAddresses attribute is populated in Azure AD by reviewing Microsoft's guide. This will help you identify which on-premises objects are generating the conflicting onmicrosoft.com addresses.
Log into the Microsoft 365 Admin Center as a Global Administrator. Navigate to the 'Support' section on the left sidebar, click 'Help & support', and open a new service request to have Microsoft engineers perform a backend tenant investigation.
Manage Your Migration Documentation with WPS Office
While you work with Microsoft Support to resolve your Azure AD proxy address conflicts, you can rely on WPS Office to manage your migration plans, server logs, and technical documentation. WPS Office is a lightweight, free alternative that offers seamless compatibility with your existing workflow.
Frequently Asked Questions
Why do unique User Principal Names (UPNs) still result in proxy address conflicts?
Even if UPNs are unique across different on-premises domains, synchronization engines like Azure AD Connect might generate identical SMTP proxy addresses in the default tenant routing domain (onmicrosoft.com), leading to a direct conflict.
How can I locate the duplicate proxy address in my local Active Directory?
You can use the Microsoft IdFix DirSync Error Remediation Tool or run custom PowerShell scripts to query your local Active Directory forest for duplicate entries in the proxyAddresses attribute before initiating the sync to Microsoft Entra ID.
What happens if a proxy address conflict is left unresolved during migration?
If left unresolved, Azure AD Connect will fail to synchronize the conflicting user attributes properly. This can lead to mail routing failures, missing mailbox aliases, or users being unable to send and receive emails in Exchange Online.
Can I manually delete the conflicting proxy address directly in Azure AD?
For accounts synchronized via Azure AD Connect, the proxyAddresses attribute is managed entirely on-premises. You must resolve the conflict within your local Active Directory and force a Delta or Initial sync to update Microsoft Entra ID.
