Greylisting is an anti-spam technique in which a receiving mail server temporarily rejects an email from an unfamiliar sender the first time it arrives, asking it to try again later. Legitimate mail servers automatically retry and are accepted on a subsequent attempt, while many spam systems never retry — so the message is quietly filtered out.
How greylisting works
When a message arrives from an unknown combination of sending IP, envelope sender and recipient, the receiving server replies with a temporary "try again later" error (an SMTP 4xx code), a form of soft bounce. A properly configured sending mail transfer agent (MTA) queues the message and resends it minutes later; the receiver then recognizes the retry and delivers it, often whitelisting that sender for a period. Unlike throttling, which slows the rate of sending, greylisting blocks the first attempt outright. The trade-off is a short delay on the first message to each new recipient.
Why greylisting matters
Greylisting is cheap and effective against low-effort spam, but it can delay time-sensitive mail such as password resets if a sender's infrastructure retries slowly or not at all. Sending from a reliable, correctly configured platform like Mailpro ensures retries happen and protects deliverability. Learn how to pass anti-spam filters, see how to avoid anti-spam filters, or explore Mailpro's anti-spam tools.
Mailpro and deliverability
Retries and routing handled for you
Mailpro sends from reliable, correctly configured infrastructure that retries and routes intelligently, so greylisting and other filters don't quietly hold back your mail.