Telnet was abandoned due to its lack of encryption, but it is still a useful tool for debugging remote connection problems. Telnet can be used to establish a connection to almost any protocol, including encrypted ones. This guide explores how to use telnet to troubleshoot connection issues. Four main errors can occur when trying to connect to a problem server, including hanging connections, DNS problems, servers not listening on a port, and connections closed by the server. Using telnet can help narrow down where the problem lies and save time in attempting to fix the issue.
https://bash-prompt.net/guides/telnet/