There are several options for Website monitoring.... like HTTP Web Server, POP3 Email Server, SMTP Outgoing Email Server, FTP File Transfer Protocol, SSL Secure Socket Layer.
If u please be specific here, provide more details ! what r the SW, OS, that can help to figure out. Though u may try out this, ServiceUptime is free website uptime monitoring SW that is designed to help detect website downtime.
I think the question is pretty clear, and I don’t see how pop3 and smtp are some of the options for website monitoring.
Most monitoring solutions have website monitoring capabilities, but they will monitor the health of your site. I don’t know of a monitoring tool that performs vulnerability scans as part of its monitoring.
However, there are many vulnerability scanners that will look for website vulnerabilities and recommend corrective actions.
You might need to find the way to combine these tools to get the type of monitoring you are looking for.