1. Check Your Internet Connection
Make sure you’re connected and not experiencing dropouts:
- Restart your router or switch to a more stable network.
- Avoid VPNs or proxies if they’re not required.
2. Flush and Reset DNS
Refreshing your DNS cache may solve resolution problems:
- Open Command Prompt as Admin.
Run the following commands one by one:
ipconfig /flushdns
ipconfig /registerdns
netsh winsock reset
- Restart your PC.
3. Update Your System Time & Date
Outlook may fail to authenticate if your device’s time is out of sync:
- Go to Settings → Time & Language and enable Set time automatically.
4. Disable Proxy Settings
Proxies can block access to Microsoft login servers:
- Go to Settings → Network & Internet → Proxy.
- Turn off Use a proxy server if enabled.
5. Whitelist Microsoft Domains
Ensure firewall or antivirus isn’t blocking:
- Add these URLs to the allowlist:
- login.microsoftonline.com
- aadcdn.msftauth.net
- secure.aadcdn.microsoftonline-p.com
6. Clear Cached Credentials
Old tokens may cause this issue:
- Go to Credential Manager → Windows Credentials.
- Remove any saved credentials related to MicrosoftOffice, MSOffice, or login.microsoftonline.com.
7. Re-add Your Account
If nothing else works:
- Open Outlook → File → Account Settings.
- Remove and then re-add your Microsoft 365 account.