What is “greylisted”?

Your recipient's server just told your email to wait. That's greylisting. It's not a rejection, and it's not your fault. It's a spam filter technique that buys the receiving server a few minutes to think.

Here's what actually happens. When your email arrives at a server that uses greylisting, the server checks three things together: your sending IP address, your sender address, and the recipient address. If it hasn't seen that exact combination before, it sends back a temporary rejection code (usually a 450 or 451) that says something like "try again later."

Your sending server will automatically retry after a few minutes, typically somewhere between 5 and 15 minutes. When it tries again with the same three details, the receiving server recognizes the combination and lets the email through. Spam bots usually don't bother retrying, which is exactly the point.

The practical upshot for you as a sender: the first email to a new recipient at that server will be delayed slightly. Every email after that passes straight through. Your ESP or mail server handles the retry automatically, so there's nothing for you to do.

The one time greylisting becomes a headache is with time-sensitive emails like one-time passcodes or password resets. A 10-minute delay on a login code can feel like a broken system even when everything is technically working. If that's happening regularly, it's worth asking your ESP whether they have specific handling for transactional sending that reduces greylist delays.

If you're seeing greylisting pop up a lot across many different recipients, it can sometimes be a signal that your sending IP is newer or unfamiliar to those servers. That's a sender reputation question worth looking into. But for a one-off greylist on a first send? That's just the internet doing its job.