I’ve been keeping an eye on the effectiveness of the blacklist that we recently installed to block spam from Communicado Ltd.

The number of messages directed at our servers seems to vary significantly, but we’ve seen close to 1,000 in a single day rejected by the filter. Whilst not a huge number in the grand scheme of things (our servers reject several connections per second using IP blacklists), it’s a pretty significant number to be spread amongst a relatively small number of customers.

What we have noticed is that the domains that we’re now seeing have been registered increasingly recently, suggesting that the older domains are becoming unusable due to people blocking them. So, we have an arms race between our ability to keep our blacklist up to date, and their ability to keep buying and deploying domains. This is actually a good thing because the domains cost the spammers at Communicado real money, so if enough people use the blacklists and keep them up-to-date then sending spam in this way will become uneconomical. In the meantime, the only consolation of the spammers chucking all this money at Nominet, is that we occassionally get to drink it.

I did some mining of our mail logs, and identified another half dozen Communicado domains. Martin fed these into his Nominet search tool yielding another 1,000 or so domains, so we’ve now got over 5,000 on the list.

If you run a mail server and aren’t already using it, please add the blacklist to your configuration and keep it up-to-date.

We’ve just put in place a new spam blacklist. What’s unusual about this list is that it’s a list of sender domains. Filtering on sender domains is not normally something we’d consider since standard operating procedure for spammers is to use a fake sender address containing a legitimate domain.

In this case, it seems that a particular company, Communicado Ltd, has gone to the trouble of registering a very large number of UK domains specifically for the purposes of spamming. We first noticed this in response to a customer complaint last week, and the thing that got my interest was that the various different domains had valid SPF records. For example:

$ host -t txt hurvabne.co.uk
hurvabne.co.uk descriptive text "v=spf1 a mx ip4:76.73.88.0/24 ip4:76.73.91.0/24 ~all"

SPF isn’t the world’s greatest anti-spam measure, but a pass on a record with specific IPs is generally a pretty positive indicator, as you have to either compromise the mail server or the DNS server – or own the domain.

What I didn’t twig was just how many domains were involved, or that they were all owned by the same company, until one of my colleagues came across Martin A. Brooks’ blog post, which identifies over 4,500 domains owned by this single outfit.

Martin has kindly shared his list of domains and we’re now filtering using it.

For what it’s worth, those domains will have cost them the best part of £17k for a year’s registration (somehow, I don’t think they’ll be renewed).