Greylisting

Most spammer's servers will try to deliver a message to the receiving server and give up if they do not get a quick response. A "real" server will retry the session after a period of time.

Greylisting allows you to reject an incoming session for a specified period of time. This will deter many spam servers from sending their messages.

Figure. General section.

Warning: For Greylisting these local bypasses are important:

Bypass trusted IPs
Exclude outgoing messages from spam scanning
Local-local bypass filter
Greylisting bypass file (greylist.dat)

If these are not applied, the users will get a temporary error 4.5.1 in their mail clients and will be allowed to send the message after x seconds.

Field	Description
Active	Check this option to enable Greylisting.
Allow new authorization after (Seconds)	Specify the amount of time that incoming connections should be rejected. Any retries within this time period will be rejected.
Expire pending sessions after (Hours)	Specify the amount of time after which any "pending" IP addresses are expired within the database. Note: "Pending" addresses are addresses which have tried to connect and have been rejected by greylisting.
Delete authorized sessions after (Days)	Specify the number of days that an authorized IP address is held in the database. A value of 0 means authorized IP addresses will never be deleted. Note: "Authorized" addresses are addresses that were rejected by greylisting, but then accepted at a later retry from the address.
Greylisting mode	Select the data that should be stored in the Greylisting database. There are four possible modes: Sender: The email address of the person sending the email. IP : The IP address of the machine sending the email. Sender&IP: Both of the above. Note: The recommended mode is Sender. Multi-IP systems, such as gmail, may retry the connection from a different IP address, and this would in turn be greylisted.
Owner mode	Choose from two options: Email: Select this option to have a greylist associated to individual email accounts. Once a message comes out of greylisting it is only accepted for that specific account. Domain: Select this option to have the greylist entry associated to the domain. So once a message passes greylisting it is accepted for the whole domain.
SMTP Response	If you wish, you can specify a custom SMTP response to be used when a connection is rejected by greylisting. If left blank, the default SMTP response message is returned.
Adaptive Mode	If enabled, it changes the way how greylisting is applied to senders. When a sender sends an e-mail, classified as a spam, greylisting is turned on for him/her. Hence, his/her next attempts are greylisted.
Bypass file (greylist.dat)	Click the B button to edit a greylisting Bypass file, where you can specify users, domains and IP address ranges that will not be greylisted. Examples are given within the file.
Greylisting	Click this button to jump to the Spam Greylist queue.