I did some work on a MTA a while back and it was genuinely pretty neat to send legit emails to my real mail accounts by just connecting to a port and typing the raw SMTP commands into it. E.g. https://en.wikipedia.org/wiki/Simple_Mail_Transfer_Protocol#...

Simpler times, when we thought we could trust everyone on the net :)

When I first started using UNIX (Ultrix on a DECStation), I was floored by the wizardry of "hackers" sending me email from Santa Claus. Mind blown!

Usenet gives full help over telnet, it's nice.