What’s a “greylist test”?
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Imagine your validation tool pings an email address to check if it's real, and the server slams the door in its face. Not because the address is bad. Just because it doesn't recognize the visitor yet.
That's greylisting. It's a spam defense technique where a mail server temporarily rejects any first contact from an unknown sender, expecting a legitimate system to retry. Most real mail servers do retry, so the message eventually gets through. Spambots usually don't bother, so they get blocked.
The problem shows up during SMTP-based validation. When a validation tool pings an address and hits a greylisting server, it gets a temporary rejection code. If the tool doesn't know what that code means, it might log the address as invalid. But it isn't invalid. The server just said "not right now."
A greylist test is what good validation tools do to handle exactly this. They recognize the temporary rejection for what it is, wait a bit, and retry. If the second attempt succeeds, the address gets marked as valid. If the tool never retries, you end up suppressing real contacts who could have received your emails.
When you're comparing validation services, it's worth asking how they handle temporary rejections. Do they retry on greylist responses? How many times? After how long? A tool that treats a "try again later" as a hard "no" is going to hand you a messier list than you started with (which defeats the whole point).
If you want to see how your current validation stacks up, we clean lists at RME and handle the edge cases like this one. Or if you just want to talk it through, the SOS hotline is free.
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