Greylisting
Greylisting is a spam-reduction technique where a receiving mail server temporarily rejects an unfamiliar sender/recipient/IP combination with a standard SMTP 4xx temporary-failure code, then accepts the message once the same sender retries after a short delay. The technique, described by Evan Harris in 2003, relies on RFC 5321's requirement that a compliant SMTP server must queue and retry temporary failures: well-behaved mail servers retry automatically, while much of the spam-sending infrastructure of the time fired messages once and moved on. Greylisting adds delivery latency, typically minutes, to first-time senders, its main downside for time-sensitive mail. It's largely a legacy technique now, since modern filtering leans more on SPF, DKIM, DMARC and sender reputation, but many mail servers still layer it on as one more low-cost check. AdminKit's MX lookup tool shows which mail server handles a domain's inbound mail so its configuration can be reviewed.
By AdminKit team, Netbase JSC · Updated