Why are open relays dangerous?

Open relays are dangerous because they allow anyone to send email through them, making them prime targets for spam, phishing, and fraud. When exploited, they can cause the server's IP address or domain to be blacklisted.

Once a server is blacklisted, legitimate emails from it may be blocked or sent to spam folders worldwide.

It is like a port that becomes known for smuggling, eventually, other harbors stop accepting its ships entirely.