Lists

Lists

When talking about email spam and our attempts to to stop it, it’s important that we’re clear about the specifics and definitions of what exactly we’re trying to stop and what we’re want to let through.

While many people will have heard of all 3 of the types of lists we will discuss on this page, there are equally as many (if not more) that haven’t. It is also important to make distinctions between the types of list (for example greylist vs blacklist) as often there are very slight, but very important, differences between the two. It is not always as obvious as the spelling out the differences between a whitelist and a blacklist. 

This page will detail what the following are and the differences between them:

  • Greylist
  • Blacklist
  • Whitelist

Table of Contents

Greylist

A greylist is a list of email sources (by that we mean IP or hosts) which have recently attempted to deliver an email to the server. While this initially may seems fine, there is a verification step involved where the sending server has to attempt to send the email one more time in order to move the host or IP from the greylist to the whitelist.

In other words it’s an in-between list. They are mail servers that are likely on their way to be moved to the whitelist however they need to pass a verification step.

This is very simplistic terms of how a greylist works. The actual implementation of how greylists are used and tested can vary and this process is automatic, not a human process.

One example of this is that a whitelist might not be a permanent list, but rather reset from time to time while the blacklist might be delivered from a 3rd party source (such as Spamhaus for example).

Blacklist

A blacklist is a list of clients that are denied access to a certain service. Typically in the case of email this relates to a list of source email addresses, IP addresses and/or hosts. 

These will all be automatically rejected by the server if they are on a blacklist.

Whitelist

In relation to email, a whitelist is a list of hosts / IPs, email addresses or servers that are automatically considered to be valid senders and therefore mail is automatically accepted from all of the sources on the whitelist.

It is important to note that when on a whitelist, no greylisting or further virus/spam checks are carried out on the mail or sender due to it’s status on the whitelist.

To be absolutely clear, the key difference between a whitelist and blacklist is that a whitelist is a list of email servers or IPs that are considered to be valid senders and automatically allows mail through to an inbox while a blacklist is the opposite. A blacklist is a list of clients that are automatically denied access to deliver mail to it’s intended destination.