What causes an IP/domain to get listed?

You didn't mean to end up on a blocklist. Almost nobody does. But blocklists don't care about intent. They care about behavior, and certain behaviors trigger automated listings fast.

Here's what actually gets IPs and domains flagged:

Hitting spam traps. Spam traps are email addresses that exist purely to catch senders with bad list hygiene. Pristine traps have never been used by a real person, so the only way you could have one on your list is if you scraped or bought addresses (a solid reason to never do either). Reactivation traps are old, abandoned addresses that were recycled. Hitting either type is a signal to Spamhaus and others that your list practices need work.

High complaint rates. When enough recipients at an ISP hit "report spam," blocklist operators notice. There's no published threshold, but even a fraction of a percent can be enough to trigger a listing on some networks. Gmail and Outlook both watch complaint signals closely.

Sending to too many unknown users. If your emails are bouncing back at a high rate because the addresses don't exist, that's a pattern blocklists recognize. It looks like a list that was guessed, scraped, or never validated. High unknown-user rates are one of the faster ways to get an automated listing.

A compromised server. If malware or an attacker gets access to your infrastructure and starts sending spam from your IP, you get listed regardless of what you intended. This isn't hypothetical. It happens, and it's why server security is part of deliverability hygiene too.

Suspicious sending patterns. Blasting huge volumes from a brand new IP, sending in sudden spikes after long silence, or sending to lists with no engagement history all raise automated red flags. Blocklist systems use behavioral signals, not just complaints.

The pattern across all of these is the same: blocklists are measuring whether your sending behavior looks like spam. Clean lists, permission-based sending, and monitoring your bounce and complaint rates are the practical ways to stay off them. None of it's complicated in theory (though keeping a list genuinely clean takes real effort).

So if you want to check whether you're already listed somewhere, our free blocklist checker scans the major networks in seconds. Worth a look before the problem finds you.