Some email recipient services use privately run spam block lists to filter incoming emails. For each email they receive, recipients ask whether the IP address of the server where the email originates is on a block list.

What do they use it for?

It is up to the recipient email provider how they act when they discover an email comes from a block-listed server:

  • reject the email unconditionally,
  • send the email to their customer’s spam folder,
  • as one factor in scoring whether an email is likely to be spam or not, or
  • lower the reputation of senders using a block-listed server (causing a much longer-term problem)

Providers who use block lists rarely give their customers or employees any choice in the matter. They don’t offer them the opportunity to make exceptions.

All mail providers suffer blocks like this from time to time, even the big ones. Generally, only smaller providers use these block lists to deny receipt of emails. They punish everyone using the blocked server, not just the problem sender (“everyone stays in after school until the culprit owns up” – a large number of different customers share most email servers).

They also make email unreliable. Usually, even smaller senders have more than one mail server. Whether it gets through then depends on the automatic choice of mail server.

Despite their serious failings, some systems do use spam block-lists unconditionally. That has a negative impact on sending mailshots until unblocked.

How does Cameo check?

Cameo checks using the same method as email providers. We check around fifty public blocklists daily. The most widely used of these blocklists, and therefore those which would have the most impact, are:

  • Spamcop
  • Spamhaus
  • Barracuda Networks

How does a server end up on a block list?

A server usually gets on a block list for one of two reasons:

  • A block list receives an email sent to a honeytrap address that they own. Such addresses can only have been obtained by illegitimate means (for example, purchasing a data breach, or by screen-scraping off web pages). They almost always indicate that someone is abusing the listed server.
  • Sufficient people report an email as spam in a way that gets back to a block list. This is pernicious: bad actors or lazy recipients can abuse it to deliberately or unintentionally trash a mail sender’s reputation. The number of reports needed is not usually very large.

The provider of the blocked server can usually apply to the block list to have the listing removed, but this may take a few days, and doing so without dealing with the source problem is likely to lead to longer and more problematic block listing in future.