Outlook Login Problems

Fix Outlook iPhone Sign-In Error AADSTS900782 for Business Email

Maira Mehtab Sep 20, 2026 868 views

Question details

The user needs to troubleshoot an authentication error code (AADSTS900782) encountered while signing into a business email account on the Outlook app for iPhone.

Product: Microsoft Outlook for iPhone
Device & OS: iPhone (iOS)
Scenario: Signing in to a Microsoft 365 business email account and synchronizing calendars on an iPhone.
Observed behavior: The sign-in process is interrupted by error AADSTS900782, resulting in a failure to authenticate the account and sync calendars.

Before you start

Ensure your iPhone has a stable internet connection and have your exact business email address and password ready before beginning the troubleshooting process.

Solution 1Recommended

Verify Account Access and Update the Outlook App

Test your business credentials in a web browser to confirm the account is active, then update your mobile app to the latest version to rule out software glitches.

Authentication errors can sometimes be isolated to the mobile application itself due to outdated software or cached data. Verifying web access helps narrow down the cause.

Test Account on the Web

Open Safari or another web browser on your device, navigate to the official Outlook on the Web login page, and attempt to sign in using your business email credentials.

Update Outlook for iPhone

Open the App Store on your iPhone, tap your profile icon at the top right, scroll down to find Microsoft Outlook, and tap 'Update' if a new version is available.

Retry App Sign-In

Launch the updated Outlook application and try the sign-in process again to see if the calendar synchronization completes successfully.

Solution 2

Document Error and Contact Administrator

Capture the exact error details for your IT administrator if basic troubleshooting does not resolve the Entra authentication block.

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Frequently Asked Questions

What does an AADSTS authentication error mean in Outlook?

AADSTS stands for Azure Active Directory Security Token Service (now Microsoft Entra). These errors occur when the authentication server blocks a sign-in attempt due to incorrect credentials, disabled accounts, or unmet conditional access policies.

Why can't my business calendars sync on my iPhone?

If you encounter a sign-in error on the Outlook app, your device cannot authenticate with the Exchange server, which immediately prevents emails, contacts, and calendars from synchronizing.

Can deleting and reinstalling the Outlook app fix this error?

Sometimes. Reinstalling the app clears out corrupt cached data that might be causing a continuous authentication loop. Ensure you have your business login credentials ready before reinstalling.

Outlook & Email Problems