Although telnet (the protocol) is outdated by ssh, the command remains quite useful. One can use telnet as a primitive tool to connect to a specific port of a specific host. This is the best method to check for connectivity to a host. The following command, for example, checks to see if it is possible to connect to the SMTP port of host mail.bigmouth.org:
telnet mail.bigmouth.org 25
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If you understand the SMTP, you can even interact with the server manually! This is because SMTP is a plain text protocol.