Greylist
From Saferpedia
Greylist is the list containing members blocked or temporarily accepted. List members can be reviewed so they'll be included in the blacklist or in the whitelist.
Operating mode
Usually a server that has a greylist will register three data components also known as triplets, for each e-mail received:
- Connecting host's IP address;
- Sender's address;
- Receiver's address.
These data are checked with the existent server's database. If the data are new for that server the e-mail is sent to the greylist for a short period of time (configurable) and refused with a temporary rejection with an error code SMTP 4xx.
The temporary rejection may be freed in different steps of SMTP dialogue, allowing the server to store the e-mail's data.
Why it works
Having a greylist on a server is efficient because many tools of spamming won't try to resend the failed messages, so the spam is not delivered.
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