Why my email id automatically changed to onmicrosoft.com instead of my custom domain. In on-prem DC domain nmae is correct. How to make the necessary changes ?

Question

Why my email id automatically changed to onmicrosoft.com instead of my custom domain. In on-prem DC domain nmae is correct. How to make the necessary changes ?

Shyam Roy 0

Why my email id automatically changed to onmicrosoft.com instead of my custom domain. In on-prem DC domain name is correct. How to make the necessary changes ?

VEMULA SRISAI 11,340 Reputation points Microsoft External Staff Moderator

2026-03-09T12:35:20.1666667+00:00
Hello Shyam Roy,

Your email/sign‑in ID changed to onmicrosoft.com because the UPN suffix from on‑prem AD does not match a verified custom domain in Microsoft Entra ID. When Entra Connect syncs a user whose UPN suffix isn’t verified in Entra ID, it automatically falls back to the tenant default *.onmicrosoft.com domain.

https://learn.microsoft.com/en-us/entra/identity/users/domains-manage

How to fix it:

In Microsoft Entra admin center → Domain names, add and verify your custom domain (e.g., contoso.com).

https://learn.microsoft.com/en-us/entra/identity/users/domains-manage

2.In Active Directory Domains and Trusts, add the same domain as an alternate UPN suffix.

https://learn.microsoft.com/en-us/entra/identity/hybrid/connect/howto-troubleshoot-upn-changes

3.Update the user’s UPN in on‑prem AD to ******@customdomain.com, then run an Entra Connect sync.

After the next sync, the user’s sign‑in ID will update back to the custom domain (authoritative change must be done on‑prem for synced users).
VEMULA SRISAI 11,340 Reputation points Microsoft External Staff Moderator

2026-03-10T14:52:07.6366667+00:00

Shyam Roy I’m following up on my previous comment as I haven’t received a response. Please let me know if you need any additional information or assistance from my side.
Shyam Roy 0 Reputation points

2026-03-13T12:03:44.0033333+00:00

Only issue with single user email, rest all users email id are accurate. Need solution for single user email id even on-prem UPN is correct for that specific user.
VEMULA SRISAI 11,340 Reputation points Microsoft External Staff Moderator

2026-03-13T12:20:00.9+00:00
hyam Roy Since this is impacting only one user, it’s usually caused by a user‑specific attribute conflict or sync condition, not the domain itself. Even if the on‑prem UPN looks correct, Entra ID can still fall back to @onmicrosoft.com when certain attributes can’t be applied during sync.

Step 1: Verify user attributes in on‑prem Active Directory

On the affected user account, check these attributes (Attribute Editor or ADSI Edit):

userPrincipalName → ******@customdomain.com

mail → ******@customdomain.com

proxyAddresses must contain:

SMTP:******@customdomain.com ✅ (Primary – uppercase)

Ensure there is no primary SMTP:******@tenant.onmicrosoft.com

If onmicrosoft.com is set as the primary SMTP, Entra ID will continue to show it.

Step 2: Check for duplicates or conflicts

Ensure the same email/UPN is not used by:

Another user

Mail contact

Guest account

Soft‑deleted user

Check Deleted users and Contacts in Microsoft Entra admin center.

Duplicate attributes force Entra ID to fall back to @onmicrosoft.com.

Step 3: Confirm the user is syncing correctly

Verify:

User is in a synced OU

Not excluded by Entra Connect filtering

User was not originally cloud‑only and later hard‑matched incorrectly

Step 4: Run Entra Connect sync

After fixing attributes on‑prem, run a sync from the Entra Connect server:

PowerShell

Start-ADSyncSyncCycle -PolicyType Delta

Show more lines

If conflicts were resolved, run Initial sync instead:

PowerShell

Start-ADSyncSyncCycle -PolicyType Initial

Show more lines

Step 5: Validate in Entra ID

After sync completes:

Check User → Properties

Confirm UPN and email now show @customdomain.com

https://learn.microsoft.com/en-us/entra/identity/hybrid/connect/howto-troubleshoot-upn-changes
Shyam Roy 0 Reputation points

2026-03-16T05:22:04.85+00:00

Already done that setting. Still the problem is remain same.
VEMULA SRISAI 11,340 Reputation points Microsoft External Staff Moderator

2026-03-19T11:19:08.9133333+00:00
Shyam Roy Thank you for confirming that all the standard attribute checks and sync steps have already been completed.

Since the issue is still affecting only this single user, this strongly indicates a backend identity synchronization conflict, most commonly related to ImmutableID (source anchor) mismatch or a hidden duplicate object in Microsoft Entra ID.

Please proceed with the following advanced checks:

Check for hidden duplicate objects Even if not visible initially, conflicts can exist in:

Deleted users (soft-deleted)

Mail contacts

Guest accounts

If any object exists with the same email/UPN, please permanently delete it, as this can block proper domain assignment.

Validate ImmutableID (Source Anchor) If the user was ever:

Created directly in cloud earlier

Deleted and recreated

Synced from another environment

There may be a mismatch between on-prem AD and Entra ID.

You can verify using PowerShell:

Get-MgUser -UserId ******@domain.com | Select OnPremisesImmutableId

If a mismatch is found, a hard match (relinking the user) may be requiredThank you for confirming that all the standard attribute checks and sync steps have already been completed.
VEMULA SRISAI 11,340 Reputation points Microsoft External Staff Moderator

2026-03-20T14:44:39.63+00:00

Shyam Roy could you please check my previous suggestions and let me know if its works for you or not?

1 answer

Your answer

VEMULA SRISAI 11,340 Reputation points Microsoft External Staff Moderator

2026-03-09T12:35:20.1666667+00:00

Hello Shyam Roy,

Your email/sign‑in ID changed to onmicrosoft.com because the UPN suffix from on‑prem AD does not match a verified custom domain in Microsoft Entra ID. When Entra Connect syncs a user whose UPN suffix isn’t verified in Entra ID, it automatically falls back to the tenant default *.onmicrosoft.com domain.

https://learn.microsoft.com/en-us/entra/identity/users/domains-manage

How to fix it:

In Microsoft Entra admin center → Domain names, add and verify your custom domain (e.g., contoso.com).

https://learn.microsoft.com/en-us/entra/identity/users/domains-manage

2.In Active Directory Domains and Trusts, add the same domain as an alternate UPN suffix.

https://learn.microsoft.com/en-us/entra/identity/hybrid/connect/howto-troubleshoot-upn-changes

3.Update the user’s UPN in on‑prem AD to ******@customdomain.com, then run an Entra Connect sync.

After the next sync, the user’s sign‑in ID will update back to the custom domain (authoritative change must be done on‑prem for synced users).
VEMULA SRISAI 11,340 Reputation points Microsoft External Staff Moderator

2026-03-10T14:52:07.6366667+00:00

Shyam Roy I’m following up on my previous comment as I haven’t received a response. Please let me know if you need any additional information or assistance from my side.
Shyam Roy 0 Reputation points

2026-03-13T12:03:44.0033333+00:00

Only issue with single user email, rest all users email id are accurate. Need solution for single user email id even on-prem UPN is correct for that specific user.
VEMULA SRISAI 11,340 Reputation points Microsoft External Staff Moderator

2026-03-13T12:20:00.9+00:00

hyam Roy Since this is impacting only one user, it’s usually caused by a user‑specific attribute conflict or sync condition, not the domain itself. Even if the on‑prem UPN looks correct, Entra ID can still fall back to @onmicrosoft.com when certain attributes can’t be applied during sync.

Step 1: Verify user attributes in on‑prem Active Directory

On the affected user account, check these attributes (Attribute Editor or ADSI Edit):

userPrincipalName → ******@customdomain.com

mail → ******@customdomain.com

proxyAddresses must contain:

SMTP:******@customdomain.com ✅ (Primary – uppercase)

Ensure there is no primary SMTP:******@tenant.onmicrosoft.com

If onmicrosoft.com is set as the primary SMTP, Entra ID will continue to show it.

Step 2: Check for duplicates or conflicts

Ensure the same email/UPN is not used by:

Another user

Mail contact

Guest account

Soft‑deleted user

Check Deleted users and Contacts in Microsoft Entra admin center.

Duplicate attributes force Entra ID to fall back to @onmicrosoft.com.

Step 3: Confirm the user is syncing correctly

Verify:

User is in a synced OU

Not excluded by Entra Connect filtering

User was not originally cloud‑only and later hard‑matched incorrectly

Step 4: Run Entra Connect sync

After fixing attributes on‑prem, run a sync from the Entra Connect server:

PowerShell

Start-ADSyncSyncCycle -PolicyType Delta

Show more lines

If conflicts were resolved, run Initial sync instead:

PowerShell

Start-ADSyncSyncCycle -PolicyType Initial

Show more lines

Step 5: Validate in Entra ID

After sync completes:

Check User → Properties

Confirm UPN and email now show @customdomain.com

https://learn.microsoft.com/en-us/entra/identity/hybrid/connect/howto-troubleshoot-upn-changes
Shyam Roy 0 Reputation points

2026-03-16T05:22:04.85+00:00

Already done that setting. Still the problem is remain same.
VEMULA SRISAI 11,340 Reputation points Microsoft External Staff Moderator

2026-03-19T11:19:08.9133333+00:00

Shyam Roy Thank you for confirming that all the standard attribute checks and sync steps have already been completed.

Since the issue is still affecting only this single user, this strongly indicates a backend identity synchronization conflict, most commonly related to ImmutableID (source anchor) mismatch or a hidden duplicate object in Microsoft Entra ID.

Please proceed with the following advanced checks:

Check for hidden duplicate objects Even if not visible initially, conflicts can exist in:

Deleted users (soft-deleted)

Mail contacts

Guest accounts

If any object exists with the same email/UPN, please permanently delete it, as this can block proper domain assignment.

Validate ImmutableID (Source Anchor) If the user was ever:

Created directly in cloud earlier

Deleted and recreated

Synced from another environment

There may be a mismatch between on-prem AD and Entra ID.

You can verify using PowerShell:

Get-MgUser -UserId ******@domain.com | Select OnPremisesImmutableId

If a mismatch is found, a hard match (relinking the user) may be requiredThank you for confirming that all the standard attribute checks and sync steps have already been completed.
VEMULA SRISAI 11,340 Reputation points Microsoft External Staff Moderator

2026-03-20T14:44:39.63+00:00

Shyam Roy could you please check my previous suggestions and let me know if its works for you or not?

Answer 1

Hey Shyam, here’s a consolidated list of all the documentation links referenced above so you can dive into the details:

Manage custom domains in Microsoft Entra ID https://learn.microsoft.com/entra/identity/users/domains-manage
Add and verify a custom domain name in Microsoft Entra ID https://docs.microsoft.com/azure/active-directory/active-directory-add-domain
Troubleshoot domain-name verification issues in Microsoft Entra ID https://docs.microsoft.com/azure/active-directory/fundamentals/add-custom-domain
Plan and troubleshoot UserPrincipalName (UPN) changes during hybrid sync https://learn.microsoft.com/entra/identity/hybrid/connect/howto-troubleshoot-upn-changes
SamAccountName issue with Microsoft Entra Domain Services (duplicate mailNicknames) https://supportabilityhub.microsoft.com/solutions/apollosolutions/5c3cceb3-6b16-45b1-c776-055e041e191e/9003261-6106
Resolve sign-in and domain-join issues in Microsoft Entra Domain Services https://docs.microsoft.com/azure/active-directory-domain-services/active-directory-ds-troubleshooting
Change your Microsoft 365 email address to use your custom domain https://learn.microsoft.com/microsoft-365/admin/email/change-email-address?view=o365-worldwide
Understand bulk user updates triggered by verified-domain changes https://learn.microsoft.com/entra/identity/monitoring-health/troubleshoot-audit-data-verified-domain

Feel free to click through any of these for step-by-step guidance. Let me know if you need anything else!

Note: This content was drafted with the help of an AI system. Please verify the information before relying on it for decision-making.

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Why my email id automatically changed to onmicrosoft.com instead of my custom domain. In on-prem DC domain nmae is correct. How to make the necessary changes ?

Step 2: Check for duplicates or conflicts

Step 3: Confirm the user is syncing correctly

Step 4: Run Entra Connect sync

Step 5: Validate in Entra ID

1 answer

Your answer