Can't sign in on phone.

For Outlook on a phone with a personal Microsoft account, try this in order:

1. Update Outlook mobile Recurring sign-in problems can happen when the app’s security standards have changed. Install the latest Outlook mobile version, then try again.
2. Try sign-in in the phone browser first Open the phone browser and try signing in to the Microsoft account there. If browser sign-in works but the app still does not move past the email page, the issue is likely with the app session rather than the account.
3. Remove and reinstall the app If the error happens in the Outlook mobile app, uninstall it and reinstall the latest version from the app store, then try again.
4. Try another network Switch from Wi‑Fi to mobile data, or from mobile data to Wi‑Fi, and test sign-in again.
5. Try another device If possible, sign in from another phone, tablet, or computer to confirm whether the problem is specific to that phone.
6. Use another sign-in method if offered If the sign-in page eventually advances, select Other ways to sign in and use another configured method such as a code, PIN, fingerprint, face, or security key.
7. Use the Microsoft sign-in helper If the account still will not sign in, use the Microsoft account sign-in helper tool. It checks the email address or phone number entered and guides the next recovery steps.

If the account is not a Microsoft account such as Gmail, Yahoo, or iCloud, sign-in help must come from that email provider instead.

References:

• Can't sign in to the Outlook mobile app | Microsoft Support
• I can't sign in to my Microsoft account | Microsoft Support
• Error 400 when signing in | Microsoft Support