What are toxicity scores?
Still have a question, spotted an error, or have a better explanation or a source we should cite?
You run your list through a validation tool, and now you're staring at a column of scores. Some addresses look perfectly valid, but their toxicity score is flashing red. What does that actually mean?
A toxicity score estimates how risky an email address is to send to, beyond whether it's technically deliverable. An address can be perfectly valid (the syntax is right, the domain exists, the mailbox responds) and still carry real risk. Toxicity scoring is how validation tools flag that gap.
Scores are built by pulling together signals from shared databases and behavioral history. Typical inputs include domain reputation, how closely the address pattern resembles known spam traps, whether the address has been tied to complaint patterns in the past, and whether the domain has shown up in blocklists or abuse reports. No single signal is definitive. It's the combination that drives the score.
High toxicity doesn't automatically mean "delete this address." It means "be careful with this one." A useful way to think about it is three buckets. Low toxicity addresses are fine to send to normally. Medium toxicity addresses are worth watching. Suppress them from cold or low-engagement campaigns, and only keep them active if you have strong evidence that person actually signed up and engaged. High toxicity addresses are ones you'd almost always want to suppress entirely, especially on shared IP infrastructure where one bad hit can affect your whole sending reputation.
The distinction between valid-but-toxic and invalid is important. An invalid address fails immediately (hard bounce). A high-toxicity address might accept the message and then quietly damage your domain reputation by triggering spam trap hits or complaint feedback loops. That's slower, harder to diagnose, and often more damaging.
If your list came from a source you didn't fully control (a tradeshow scan, a purchased append, an old import), toxicity scores are one of the best signals for figuring out which addresses to suppress before you ever touch send. And if the scores are high across a large chunk of your list, that tells you something important about the source itself.
If you want a professional eye on what your toxicity breakdown actually means for your sending, our SOS hotline is free and we actually pick up.
Contributors
Who worked on this answer
Every name links to their profile. Every company links to their site. Real people, real accountability.
