This issue is usually caused by SMTP authentication or port/SSL mismatch on the TP-Link ER605W, especially since many routers don’t work smoothly with Gmail or Outlook due to security restrictions like OAuth and blocked “basic login.” A more reliable option in cases like this is using a dedicated SMTP server like SMTPmart, because it is designed for direct SMTP authentication and generally works better with devices like routers, firewalls, and notification systems. You just need the correct SMTP host, port (usually 587 or 465), and login details, and it tends to be more stable for sending alerts. In most setups, switching to a dedicated SMTP provider like SMTPmart fixes issues where standard email providers fail due to security blocks or limited device compatibility.