What is Greylisting?
Greylisting is a method of defending electronic mail users against e-mail spam. A mail transfer agent which uses greylisting will “temporarily reject” any email from a sender it does not recognize. If the mail is legitimate, the originating server will try again to send it later, at which time the destination will accept it. If the mail is from a spammer, it will probably not be retried. When a mail server is greylisted, the duration of time between the initial delay and the re-transmission is variable. Some mail servers use a default of 4 hours, though most will retry sooner. Most open-source MTAs have retry rules set to attempt delivery after around fifteen minutes. Our servers do not currently use greylisting, although many recipient servers (e.g. BT) will. http://en.wikipedia.
Greylisting is a method of protecting you from spam and viruses that relies on systems correctly complying with the rules for sending email, and is very effective at preventing spam (particularly when used in combination with our other techniques). There are a couple of downsides however. Firstly, not all systems on the Internet correctly comply with the rules for sending email even when they have a legitimate need to send email. Secondly, greylisting can slightly slow down email, particularly the first time you receive email from a new correspondent. Typically the delay introduced is only 5 minutes or so, the first time you receive an email from a new contact, but it can be higher and depends on the system sending email to you, not our email scanning system. Overall, we highly recommend greylisting for all customers. But be aware that, on the first email from a new contact, a delay will be introduced.
Greylisting is simple and powerful and it works because a message rejected with a temporary failure code will be queued by a true SMTP server and the delivery will be attempted again usually within 15 minutes. A compromised Windows machine sitting on a kitchen table somewhere will try to send a message a few times in succession and then move on to the next spam victim’s email address. The first time we see an IP address/sender/recipient tripple, and the sender/server meets one of the criteria for Greylisting, the message will be rejected with a temporary error code. A message from an SMTP server that attempts delivery 5 minutes or more after the first delivery attempt to the same IP address/sender/recipient tripple will be accepted. Greylisting a true queue and retry SMTP server serves no useful purpose. Once a message has been accepted, further messages from that IP address will not be Greylisted for 30 days. Several times daily software analyses the Greylist database performing tests
Greylisting consists in refusing temporarily a message. If the issuer is a real mail server (ie. specific to the SMTP RFC) it will necessarily be resent then accepted. If the issuer is a spammer it will rarely be forwarded. This is the reason why ALTOSPAM uses this technology when the nature of the message is ambiguous. In return this technique generates a latent period in the reception of emails. (Cf. article on greylisting).
Greylisting is the name of a method that is used to reduce the delivery of junk e-mail but allow legitimate e-mail messages to be delivered. Greylisting looks at the IP address of the sender and the recipient address. If the system does not recognize this information, then it will temporarily reject the e-mail. All legitimate e-mail systems will attempt to resend the e-mail at a later time, and the e-mail will go through. Most applications that send junk e-mail just send junk e-mail messages once and do not attempt to resend them. These junk e-mail applications use the “fire-and-forget” method, which means that if the junk e-mailer was unsuccessful at sending junk e-mail to a particular e-mail address, the application will not try to resend that e-mail message. Greylisting does not delete legitimate e-mail messages. All legitimate e-mails will go through using greylisting. The only potential effect greylisting may have for pair Networks customers is a slight delay in the delivery of le
