What is “IP reputation block”?
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You hit send, and instead of a delivery confirmation, you get a bounce back saying your IP is blocked. That's an IP reputation block, and it means the receiving server has looked at your sending IP address's history and decided it doesn't trust you.
An IP reputation block happens when a receiving mail server refuses your email because your sending IP has a poor track record. Servers make this call automatically, often in milliseconds, based on signals like past sending behavior and whether the IP appears on any known blocklists.
A few things can push an IP into this territory. High complaint rates, sending to spam traps, emailing lots of invalid addresses, or simply having been used for spam in the past (even by someone else if you're on a shared IP) can all leave a mark. The reputation follows the IP, not the sender name.
How do you know if this is what's happening? Your bounce message will usually reference the IP address directly, mention "sender reputation," or include a link to a blocklist lookup. You can also check your IP's standing with Twilio SendGrid's Sender Score tool or use Spamhaus to query the major blocklists. Postmark's free MTA debugging tools can also help you read the rejection reason clearly.
If you're already blocked, the path forward looks something like this. Temporarily reduce your sending volume. Scrub your list to remove invalid addresses and anyone who's complained. Then monitor your engagement to show receiving servers that real people are opening your mail. If a specific blocklist is the culprit, you'll need to submit a removal request directly with them. In more serious cases, warming a new IP might be the faster route to recovery.
Prevention is simpler than recovery. Keep your list clean, watch your complaint rates, and warm any new IPs gradually before sending at full volume. You can run a quick blocklist check on your domain and IP with our free blocklist checker. If you're dealing with a block right now and not sure where to start, our SOS hotline is free and we'll help you figure out the fastest path out.
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