What is an open relay?
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An open relay is a mail server that accepts and forwards email from anyone to anyone, without checking whether the sender is authorized to use it. In the early days of email, this was how most servers worked. Today, it's a security disaster.
Why? Because spammers search the internet for open relays, then use them to blast millions of spam emails. The spam appears to come from your server's IP address, not theirs. Your server gets blocklisted, your legitimate emails stop delivering, and you're cleaning up someone else's mess.
Modern mail servers require authentication before they'll relay mail. That's SMTP AUTH (you log in with a username and password), or they only relay for specific IP addresses (like your company's network). If you're using an ESP like Mailchimp or Postmark, you don't need to worry about this. They handle authentication for you.
Open relays are mostly a problem if you're running your own mail server. If you manage a server running Postfix, Exim, or Sendmail, you need to verify it's not an open relay. Test it by trying to send email through your server from an external network (not your office) without logging in. If it sends, you have an open relay. Fix it by requiring SMTP authentication for all relay attempts.
And If you're not sure whether your server is configured correctly, MXToolbox's SMTP diagnostics can test it for you. Or just ask us (seriously, we'll check it for free).
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