Greylisting

You can use greylisting to identify potential spam messages by temporarily rejecting messages from unknown senders. Greylisting reduces spam because, while legitimate senders will resend messages if requested, spam sources usually do not.

To enable greylisting, the Gateway maintains:

A record of known senders. Recognized senders who have sent email to the Gateway within a specified period. By default, if a sender has not sent a message to the Gateway in more than 36 days, they are removed from the record of known senders.
A record of greylisted senders. Unrecognized senders who have had a message temporarily rejected by the Gateway. By default, if a sender does not retry the message within 24 hours, they are removed from the record of greylisted senders.

This table illustrates how greylisting handles messages:

Is the sender in the record of known senders?	Is the sender in the record of greylisted senders?	Actions
No	No	Message Greylisted The Gateway creates a greylist record for the sender, and rejects the message with a temporary failure for, by default, five minutes.
No	Yes	Message Accepted If the time period for temporary failure has expired, the Gateway removes the sender's greylist record, adds the sender to the list of known senders, and accepts the message.
Yes	N/A	Message Accepted The Gateway updates the time that a message was last received from the sender, and accepts the message.

Is the sender in the record of known senders?

Is the sender in the record of greylisted senders?

Actions

Message Greylisted

The Gateway creates a greylist record for the sender, and rejects the message with a temporary failure for, by default, five minutes.

Yes

Message Accepted

If the time period for temporary failure has expired, the Gateway removes the sender's greylist record, adds the sender to the list of known senders, and accepts the message.

Yes

N/A

Message Accepted

The Gateway updates the time that a message was last received from the sender, and accepts the message.

See Configuring Greylisting Settings for information about enabling greylisting in your spam policy.

See Configuring Advanced Greylist Settings for information about changing the periods for which messages are rejected, greylist entries are kept, and records of known senders are kept.

Greylisting can only be used as an anti-spam defense if the Gateway is connected directly to the Internet enabling communication with unrecognized senders.

To avoid messages from trusted sources being greylisted, add them to a white list.

Only senders with Suspicious or Bad TRUSTmanager reputations can be greylisted.