Outlook Mobile App authentication error Exchange 2019

Hi Ali Raza

Thank you for reaching out to Microsoft Q&A forum

This behavior typically points to the Outlook mobile sign-in flow being unable to complete against your Exchange publishing configuration (most commonly Exchange update level / Extended Protection compatibility, TLS + certificate trust, or Autodiscover/ActiveSync reachability).

And because you mentioned that only a subset of users (10–15) are affected, so it often indicates an inconsistency that only some users/devices hit these configurations.

Therefore, in this context, you can follow the below instructions to see if it can help you:

1. Validate TLS 1.2 and certificate trust end-to-end
   • Outlook for iOS/Android is sensitive to TLS configuration and certificate validity/trust. Confirm the certificate is valid, matches the published namespaces, and is trusted by the mobile devices (public CA or properly deployed internal CA).
2. Test Autodiscover + Exchange ActiveSync externally
   • Run the Microsoft Remote Connectivity Analyzer tests (Autodiscover + ActiveSync) using an affected user. If the test fails, correct the indicated DNS/certificate/connectivity issue first; if it passes, proceed to client-side remediation.
3. Client-side reset for only the impacted users
   • On affected devices: remove the account from Outlook mobile, then re-add it after server-side checks are completed (this helps clear cached config/credentials).

Hope my answer will help you. For any further concern, feel free to let me know in the comment section.

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Use the following checks and fixes for the affected Exchange Server 2019 users whose Outlook for iOS/Android profile setup hangs on “Authentication in progress” and then shows an authentication error.

1. Confirm Exchange 2019 is fully updated

• If Extended Protection was enabled on Exchange 2019/2016 and the server is not on a current Cumulative Update, Outlook configuration on Android/iOS can fail with a generic error.
• Install Exchange Server 2019 Cumulative Update 13 or a later CU on all Exchange 2019 servers, then retry profile setup.

2. Verify TLS 1.2 and certificate health

• Outlook for iOS and Android will not connect if the environment is using TLS 1.0/1.1 or if the TLS certificate is invalid/untrusted.
• Ensure:
  • The Exchange 2019 servers and their OS are configured to use TLS 1.2 or later only.
  • The SSL certificate used by Exchange is valid (not expired), matches the namespaces in use, and is trusted by standard public CAs (or otherwise trusted by the device). If the certificate is untrusted or invalid, Outlook mobile can fail during authentication.

3. Check for certificate trust warnings on mobile

• If users see “Insecure connection” or “Secure connection unavailable” when adding the account, this indicates the certificate is considered unsupported (invalid/expired or untrusted).
• In that case:
  • Replace or renew the certificate with one signed by a CA that is trusted by the standard Java CA store.
  • After updating the certificate, have affected users remove and re-add their account in Outlook mobile.

4. Validate Autodiscover and ActiveSync externally

• Use the Microsoft Remote Connectivity Analyzer (Exchange ActiveSync Autodiscover test) against an affected user:
  • If the test fails, fix Autodiscover (DNS records, certificates, or connectivity) as indicated by the test results.
  • If the test passes, proceed with client-side remediation.

5. Client-side remediation steps

• On each affected mobile device:
  • Remove the Exchange account from Outlook mobile.
  • Fully uninstall Outlook (and Teams if tightly integrated) from the device.
  • Reboot the device.
  • Reinstall Outlook from the app store and add the account again.
• This has resolved similar mobile sign-in issues where an “unexpected error” or persistent auth problem occurred only for a subset of users/devices.

6. Check for recent Windows/Kerberos updates (on-premises only)

• If the environment uses Kerberos for authentication and November 2021 (or later) security updates were applied to domain controllers, install the corresponding out-of-band Windows updates on DCs to fix known Kerberos issues that can break Outlook and mobile client authentication.

After applying server updates (Exchange CU and any required DC hotfixes), confirming TLS 1.2 and certificate health, and reinstalling Outlook on affected devices, retry the account setup. If only a small subset of users remains affected, compare their mailbox configuration and device OS versions with working users to identify any additional device- or user-specific differences.

References:

• Outlook configuration fails in Android or iOS
• "The connection to your mail server timed out" error in Outlook for iOS or Outlook for Android
• Error when Outlook for iOS or Android connects to Exchange Server: Insecure connection or Secure connection unavailable
• A mobile device can't connect to Exchange Online by using Exchange ActiveSync
• Outlook and mobile clients don’t authenticate after November 2021 fixes for Windows installed
• Correlation Id: [Moderator note: personal info removed] Timestamp: 2025-11-06T00:32:06.000Z DPTI: [Moderator note: personal info removed] 7 Message: An unexpected error occurred. Tag: 4oggx Code: 1864098253 - Microsoft Q&A