Userbase

Many, even most, of those services will be appropriate. For instance, a web server with an open port 80, the “door” through which HTTP web pages are sent through, is appropriate (even if the encrypted version, HTTPS, would be more secure). But eight of the top 10 services offered by servers on the internet are unencrypted, such as POP3, an outdated email protocol, and FTP, an insecure method of transferring files over the net.