What is “relay denied”?
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You hit send, and an hour later you get a bounce message that says "relay denied." That's the receiving mail server saying your sending server didn't have permission to deliver that message. It's a rejection, not a delivery failure. Here's why it happens.
Your server didn't authenticate properly. Every mail server checks: "Is this message actually from who it claims to be?" If your Sender Policy Framework (SPF) record doesn't include your sending IP, or your DKIM signature fails, the receiving server treats you as untrusted and blocks the relay.
Or your server isn't configured right. Maybe you're sending from an IP that wasn't authorized. Maybe the DKIM key is misconfigured or expired. Maybe your domain's SPF record is too restrictive and doesn't mention your mail provider. These are all relay denial triggers.
Here's what happens next. The message doesn't bounce temporarily. It's rejected outright. If you keep sending without fixing authentication, mailbox providers notice and your sender reputation drops. One relay denial might not hurt. Ten thousand? That's a problem.
Fix a relay denied error by checking three things: your SPF record (does it include all your sending IPs?), your DKIM setup (is the key valid?), and your sending server configuration (are you using the right authentication credentials?). Start with our free SPF Checker to validate your record. If the error persists, our SOS team can walk you through DKIM setup and server config troubleshooting.
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