ESMTP Authentication protocol error - Error code 25 This error can occur when the Microsoft SMTP server starts up on port 25. An SMTP authentication Error code 25 typically occurs because of a conflict between MailSite and the Microsoft SMTP Service on port 25, often after the server has been rebooted. The error is produced because the end user is attempting to authenticate on the Microsoft SMTP server instead of MailSite. To avoid this error, shutdown and disable the Microsoft SMTP Service through the Services control panel.