Greylisting

Greylisting is a mechanism that supplements the existing SCS Sophos antispam service to further reduce unwanted email. The Sophos/Greylisting combination has demonstrated its effectiveness in minimizing spam that reaches either a user's INBOX or SPAM folders.

How does Greylisting work?

Greylisting examines both sender and recipient of a message. The first time our incoming servers see a message from a particular host and sender for a local recipient, they ask the sending host to try again later. Further messages with that host/sender/recipient triplet are delivered normally.

Spammers typically use pools of multiple machines to send out large volumes of messages to users. Since they try to send the maximum number of messages as quickly as possible, those that fail the first time are usually not retried; they are simply dropped. Tests have confirmed this pattern: Over 90% of the time, spam messages are not resent.

Will this affect my normal (nonspam) mail?

To a small extent, yes. However, if you correspond regularly with a particular user, only the first message from that user will be affected, and then only slightly. That initial message will be resent by the sending machine (typically in under ten minutes). Subsequent messages from that user will not be affected.

What about mail from CMU people?

Mail from CMU hosts is not subject to Greylisting.

How long does Greylisting take to start?

Greylisting can be turned on very easily and takes effect immediately. Users can also disable it themselves at any time.

How do I sign up?

New user accounts (and mailing lists) will have Greylisting enabled by default. Current users can sign up on the Corvid attribute lookup page by setting the "Grey Listing" flag. Alternately, you can ask the HelpDesk staff to set it for you. SCS Facilities will reconfigure all current mailing lists to activate Greylisting. Contact the SCS HelpDesk, <help+@cs.cmu.edu> or x8-4231, or stop by GHC 4203, with any questions or problems.