What Is Greylisting?
Greylisting is an anti-spam technique where a receiving server temporarily rejects email from an unfamiliar sender with a "try again later" response. Legitimate mail servers retry and are then accepted, while many spam systems never retry — filtering out low-effort spam at the cost of a short delivery delay.
The first time a server sees a new sender/recipient/IP combination, it returns a temporary failure (a 4xx soft bounce) and records the attempt. A properly configured sending server retries after a few minutes, and greylisting then lets the message through and often whitelists the sender for future mail.
The main side effect is delay — a first email from a new sender may take minutes to hours to arrive. This is usually harmless for outreach but worth knowing when diagnosing why an initial message seems slow.
Because greylisting relies on retry behavior, it rewards well-configured, standards-compliant senders and penalizes throwaway spam infrastructure that fires once and moves on.
MailPilot sends from properly configured mailboxes with correct retry behavior, so greylisting resolves automatically and your warmup traffic is accepted reliably.
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