What is a block bounce?
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You sent an email, it bounced, and the bounce message says something like "550 5.7.1 Message rejected" or "554 rejected due to policy." The address is real. The server is running. But your message didn't get through. That's a block bounce.
A block bounce happens when the receiving server actively refuses your message. Not because the address doesn't exist, and not because something broke on their end. They know who you are, and they've decided they don't want your mail.
Block bounces are different from other bounce types. A hard bounce means the address is gone. A soft bounce is usually a temporary hiccup on the recipient's side. A block bounce is specifically about you. Your IP, your domain, your content, or your authentication setup triggered a refusal.
The most common reasons a block bounce happens:
- Your sending IP is on a blocklist. Services like Spamhaus or Barracuda flag IPs with a history of spam. If your IP lands on one, receiving servers that check those lists will block you.
- Your domain has a poor reputation. Mailbox providers track how recipients interact with your email over time. High complaint rates, low engagement, and spam trap hits all drag your reputation down.
- Your content triggered a filter. Certain subject lines, link patterns, or body copy can look spammy to filters even if your reputation is otherwise fine.
- An authentication failure. If your SPF, DKIM, or DMARC records aren't set up correctly, some servers will reject your mail outright based on policy.
The important thing to know: block bounces aren't always permanent. Unlike a hard bounce where the address is truly dead, a block bounce often reflects a reputation problem that can be fixed. That said, it does require actual investigation. Suppressing the address and moving on won't solve anything, because the same block will hit every message you send to that server.
The causes of block bounces can stack on top of each other too. You might have a reputation issue AND a content issue, and both need to be addressed before delivery improves. If you're seeing block bounces across multiple domains or providers, that's usually a signal that something systemic is going on with your sending setup.
Not sure if you're on a blocklist right now? You can run a free check with our blocklist checker. If things feel more urgent than that, the SOS hotline is free and we actually help (no pitch, just answers).
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