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 ip4:188.8.131.52/24 ip4:184.108.40.206/24 ~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).