To repair a QuickBooks Desktop Webmail password error, first, ensure your email settings are correctly configured. Go to Edit > Preferences > Send Forms, and select your email provider. Verify your email address, server details, and port settings. If the password error persists, try updating QuickBooks Desktop to the latest version. If that doesn't resolve the issue, reset your email password, and update it in QuickBooks. Additionally, temporarily disable antivirus software, as it might interfere with QuickBooks' email functionality.