RFC 6647 / START HERE
Email greylisting, before the details
Greylisting is a name for temporarily reduced service to unfamiliar mail senders; SMTP means Simple Mail Transfer Protocol.
A receiving mail server can return a temporary failure and remember an attempt. A sending server keeps the mail queued and tries again; a qualifying retry can pass the greylisting check.
The idea in one picture
Defer an attempt
Remember history
Recognize a retry
A simplified picture of the idea; the chapters explain the actual mechanisms.
Why it exists
RFC 6647 is a June 2012 Proposed Standard describing where greylisting can be applied, useful retry history, operational recommendations and costs. It changes no SMTP command and has no formal updates or obsoletions.
A familiar example
Imagine a reception desk asking an unfamiliar courier to return later and recording the visit. Returning can satisfy that waiting rule, but it neither proves the courier honest nor guarantees the parcel is accepted. Legitimate urgent parcels can be delayed too.
Helpful background
You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.
What you’ll learn next
Begin with chapter one, or choose a topic.
- Defer an unfamiliar sender, then recognize a retryGreylisting uses an ordinary temporary SMTP failure to ask an unfamiliar sending server to try later.→
- Choose a key from information already availableA greylisting key is remembered information about an attempt; it depends on the SMTP phase.→
- Track the retry window separately from record expiryThe age of an attempt and the lifetime of known-source history answer different questions.→
- Keep retry identity consistent across mail serversChanging the receiving server, sending address or key can make the same mail look new.→
- Apply exceptions without calling them proof of safetySome legitimate senders should avoid this delay, but exceptions have costs and limits.→
- Measure the trade-off and bound the claimsA policy should be assessed against both abusive traffic and legitimate mail costs.→
This introduction explains RFC 6647 (2012). Read the original RFC ↗