> What was the original intent of open relays? Why allow emails without authentication?

Store and forward.

Do remember that email was THE great federated protocol.

The goal of a mail server was to get your email "at least one hop" closer to your destination. And that wasn't an easy task.

Servers came online and went offline. Users logged in and out. Connections came up and went down. IP wasn't the only transit. DNS? Oh, the hosts file? Even higher things--thing DECnet and Janet.

Email was barely functional most days. Your best bet if you weren't an Internet God and weren't able to write your own super complicated sendmail.cf was to know a sysadmin at a node who had an Internet God and ask him if you could forward emails that you couldn't handle to their server.

Email would be so amazing were it not for the spam problem. In the early days you’d just send a mail to your computer and your address was yourlogin@yourdomain and mail just ended up on your machine in a folder. Relays were like p2p networks. It was actually beautiful in its simplicity and in a perfect world with everyone being good actors could have been incredible.

And it was, back when any hint of "commercial use" could get your machine booted from the mail routes and usenet. After Cantor and Siegel, it was every spammer for himself.

Open relays were offered in the spirit of cooperation that was characteristic of the early internet.

Unfortunately, greedy people soon jumped in to take advantage of this generosity, resulting in a tragedy of the commons.

John Gilmore used to run an open relay, and I used to get spam from it. He was really stubborn about promoting the freedom of the spammers over the peace and quiet of the poor recipients. He eventually got shut down, still complaining.

http://www.toad.com/gnu/verio-censorship.html

> Why allow emails without authentication?

Because the people on the 'net were generally not bad actors back in the day, so we would you need to lock things down?

Hmm, now the internet is a dark forest. Leave anything open/visible, and all the bad actors immediately swamp you.