User-1281870346 posted
Set the custom error for 401 to invoke an ASP page that sends you an email. Note that if you are seeing hundreds of attempts, you will get hundreds of emails.
Update; Sorry, I just realized that the subject says FTP. I don't know of any real time notification for that. You could use a scheduled task that wakes up ever few minutes and reads the FTP log and emails you a summary report.