This anti-spam feature is available on all Cobourg Internet web sites although it is generally disabled by default.  The version used is the one supplied by CPanel.  It is only turned on if:

  • Spam received by email accounts on the site is unusually severe.
  • The emails are not used as the primary email for an individual (see disadvantages below).

How does it work

A server employing greylisting deliberately degrades mail service for unknown or suspect sources, over a short period of time. It records three pieces of data, known as a "triplet", for each incoming mail message:

  • The IP address of the connecting host
  • The envelope sender address
  • The envelope recipient address(es), or just the first of them.

This data is registered on the mail server's internal database, along with the time-stamp of its first appearance. The email message will be dismissed with a temporary error until 10 minutes has elapsed. Most legitimate mail senders will automatically retry – typically after 15 minutes. When a sender has proven itself able to properly retry delivery, it will be whitelisted for a longer period of time, so that future delivery attempts will be unimpeded. Cobourg Internet requires a successful delivery attempt against a registered triplet to be no earlier than 10 minutes after registration and not later than 4 hours after it. Repeated delivery attempts before the 10 minute period will be ignored with the same error reply code. After 4 hours the triplet will be expired, so delivery attempts will register anew. When the greylister sees an attempt within the 10 minute - 4 hour window, the connecting host will be whitelisted for 6 days.

In addition to whitelisting good senders, Cobourg Internet provides for exceptions. Many senders known to strongly ban spammers from their network are listed as Trusted Hosts.  This includes Microsoft, Google, Yahoo, Constant Contact, Mail Chimp and some others.

Why it works

Greylisting is effective because many mass email tools used by spammers do not queue and reattempt mail delivery as is normal for a regular Mail Transport Agent. They do not queue and resend email as this requires the expenditure of resources (spamming normally operates on very narrow margins).


The main advantage from the users' point of view is that greylisting requires no additional configuration from their end. If the server utilizing greylisting is configured appropriately, the end user will only notice a delay on the first message from a given sender, so long as the sending email server is identified as belonging to the same whitelisted group as earlier messages


  • There is a delay in receiving email
  • Some legitimate mail does not resend – e.g. newsletters – these would be lost.  Note that Mail Chimp or Constant Contact are whitelisted so would get through..
  • Emails that are sent when you click ”I forgot my password” do not resend so would be lost.
  • Sometimes for some senders, the retry comes from a different IP address – these would be lost.


Overall greylisting significantly reduces spam although there is a risk that some wanted mail would be lost and many emails would be delayed.

The standard spam and virus protection is enabled for all uses, including anyone who has enabled greylisting.  This standard protection is generally good enough for most users.

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