Outlook Desktop fails with "Email address can't be found" (Error 500) but OWA and Mobile work fine

Question

Hi everyone,

We are facing a specific issue with some users who were recently migrated from Exchange On-Premise to Exchange Online in a Hybrid environment.

The Symptom:

• Users can successfully log in and use OWA (Outlook Web App) and Outlook Mobile.

Outlook Desktop (Microsoft 365 Apps) fails to set up the profile, claiming the email address cannot be found.

New users created directly in the cloud do not experience this issue; it only affects migrated (synced) accounts.

Microsoft Connectivity Analyzer (RCAT) Results: The test fails during the Autodiscover POST request with the following details:

ErrorCode: 500

Message: The email address can't be foundHi everyone,

We are facing a specific issue with some users who were recently migrated from Exchange On-Premise to Exchange Online in a Hybrid environment.

The Symptom:

Users can successfully log in and use OWA (Outlook Web App) and Outlook Mobile.

Outlook Desktop (Microsoft 365 Apps) fails to set up the profile, claiming the email address cannot be found.

New users created directly in the cloud do not experience this issue; it only affects migrated (synced) accounts.

Microsoft Connectivity Analyzer (RCAT) Results: The test fails during the Autodiscover POST request with the following details:

ErrorCode: 500

Message: The email address can't be found.

HTTP Response Headers: X-CalculatedBETarget: AM6PR09MB2551.eurprd09.prod.outlook.com

Note: The request is hitting the correct Exchange Online backend, but the service returns a "not found" error for the mailbox object.

Steps Taken So Far:

Applied Registry fixes: ExcludeScpLookup, ExcludeHttpsRootDomain, and ExcludeExplicitO365Endpoint (set to 0).

Cleared Credential Manager and disconnected accounts from "Access Work or School".

Verified that Modern Auth (ADAL) is enabled.

Confirmed that MapiEnabled and MapiHttpEnabled are set to $true for the affected users.

It seems like an identity mismatch or an ExchangeGuid / ImmutableID desync between Entra ID and Exchange Online after the migration. Has anyone encountered this "Object not found" behavior specifically for Autodiscover while other protocols work?

Any insights or suggestions would be greatly appreciated.

Answer 1

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Hi Muhammet,

Thank you for reaching out to Microsoft Q&A. After moving the mailbox from on-premises Exchange to Exchange Online, the ExchangeGuid stored in the on-premises RemoteMailbox object (synced via Entra ID Connect) no longer matches the actual mailbox GUID in Exchange Online.

Outlook Desktop and the Microsoft Connectivity Analyzer rely on the Autodiscover POST request to https://autodiscover.outlook.com/.... This request performs an internal directory lookup using the SMTP address together with that ExchangeGuid. When the GUIDs are out of sync, the Exchange Online backend simply returns Error 500 – “The email address can't be found” (you can see in the response headers that it still hits the correct server via X-CalculatedBETarget).

OWA and Outlook Mobile continue to work perfectly because they authenticate directly against the user object in Entra ID and the mailbox in Exchange Online. Cloud-native mailboxes never experience this problem at all, since they have no on-premises RemoteMailbox object.

That’s exactly why only the migrated (synced) accounts are affected.

First let have a quick check:

In Exchange Online PowerShell (Connect-ExchangeOnline):

Get-Mailbox ******@contoso.com | Select DisplayName, PrimarySmtpAddress, ExchangeGuid

In On-Premises Exchange PowerShell:

Get-RemoteMailbox ******@contoso.com | Select DisplayName, PrimarySmtpAddress, ExchangeGuid

If the two ExchangeGuid values do not match >>> you’ve confirmed the root cause. to fix it (one-time per user)

1. Copy the ExchangeGuid from the Exchange Online mailbox.
2. Update the on-premises RemoteMailbox: PowerShell

      Set-RemoteMailbox ******@contoso.com -ExchangeGuid <Paste-EXO-GUID-here>
   

   1. Force an Entra ID Connect delta sync (on your AAD Connect server):
   PowerShell

      Start-ADSyncSyncCycle -PolicyType Delta
   

3. Wait 5–15 minutes, then on the client either delete the Outlook profile (or just the .ost) or use Ctrl + right-click on the Outlook tray icon >> Test E-mail AutoConfiguration to verify the POST now succeeds.

The profile should now create without any issues. For Bulk Users, you can export the correct GUIDs from EXO and run a small script to update all RemoteMailboxes, followed by one delta sync.

Hope this help.

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