Non-Office 365 accounts repeatedly request sign-in each time Outlook (new) is opened or restarted

1. Quick fixes
Delete and re-add the account
File → Account Settings → select the problem account → click “Delete”
Re-add the account after restarting Outlook
2. Clear Credentials Cache
Open Windows Credential Manager (control.exe /name Microsoft.CredentialManager)
Delete all credentials related to Outlook.
3. Common causes and targeted solutions
Scenario A: Outdated authentication methods
Ensure that the account uses modern authentication (OAuth2):
File → Account Settings → select the account → click “Change”
Check “Use modern authentication” (if option is available)
 Scenario B: Third-party mail service restrictions
Check if the mail provider supports OAuth2:
Gmail/Yahoo etc. need to enable “Allow insecure applications” on the web side
Enterprise mailboxes need to contact IT administrator to open API privileges
4. Registry Repair (Windows)
Force modern authentication to be enabled:
Press Win+R and type regedit
Navigate to:
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Identity  
New DWORD value:
Name: EnableADAL
Value: 1
5. Special configuration for enterprise environment
Group Policy Adjustment:
Administrator Run gpedit.msc
Navigate to:
Computer Configuration → Administrative Templates → Microsoft Outlook 2016 → Account Settings → Accounts
Enable:
“Force use of modern authentication”
“Disable ADAL” is set to Disabled
6. Alternatives
 Use application-specific passwords (e.g. for Google accounts):
Generate a 16-digit dedicated password in the backend of your email provider
Use this password instead of the master password for Outlook logins