Deliverability
What is Greylisting?
An anti-spam tactic where a server temporarily rejects mail from unknown senders and accepts it only on a retry.
Greylisting is a server-side technique that temporarily defers the first delivery attempt from an unfamiliar sender triple (IP, envelope sender, recipient) with a 4xx try-again-later response. Legitimate mail servers retry after a delay and are then accepted, while many spam tools never retry and give up. The trade-off is added latency on first contact, typically from a few minutes to an hour.
Why it matters for cold email
Cold email frequently reaches unfamiliar recipients behind greylisting servers, so first messages can be delayed rather than failed, which skews the timing of opens and replies. A properly configured sending platform that honors retry rules passes through greylisting cleanly, whereas one that does not retry will silently lose first-touch emails.
Related terms
Stop guessing at deliverability.
Scheduler Zero handles authentication, warmup, and sender reputation so your cold email lands in the inbox, not the spam folder.