Not really hacking, but in the 90s you could usually just connect to a mail server and it would believe what you told it.

If you were careful you could just type an email directly: MAIL FROM, RCPT TO, etc.

I would write scripts at work to send spoof emails sometimes, you could put anything as the FROM address, like "info @ catfacts" or whatever.

Another "not really hacking" example is that when some companies first got an Internet connection, they would just allocate public IP addresses to everyone, no gateway or firewall. So you could browse any non-passworded smb shares just knowing the IP.