How many blocklists do we need?

There’s been a discussion on the mailop list about the number of different blocklists out there. There are discussions about whether we need so many lists, and how difficult the different lists make it to run a small mail system (80K or so users). This discussion wandered around a little bit, but started me thinking about how we got to a place where there are hundreds of different blocklists, and why we need them.
shield
There is a lot of history of blocklists, and it’s long, complicated and involves many strong and passionate personalities. Some of that history is quite personal to me. Not only do I remember email before spam, I was one of MAPS’ first few employees, albeit not handling listings. I’ve talked with folks creating lists, I’ve argued with folks running lists. For a while I was the voice behind a blocklist’s phone number.
The need, desire and demand for different lists has come up over the years. The answer is pretty simple: there are many different types of abuse. One list cannot effectively address all abusive traffic nor have policies that minimize false positives.
Lists need different policies and different delisting criteria. The SBL lists based on volume of email to addresses that are known to have not opted in to receive mail. The PBL lists IPs where the IP owner (usually an ISP) says that the IPs are not supposed to be sending mail by their policy. URIBL and SURBL list domains, not IPs. Some lists have delisting requirements, some let listees remove themselves.
The policies of listing and delisting are not one size fits all, nor should they be.
There are two widely used lists that have significantly different delisting policies: the SBL and the CBL.
The SBL focuses on IP addresses they believe are under the control of or supporting the services of spammers. They measure this by primarily relying on spamtraps, but they also accept forwarded mail from some trusted individuals. Getting delisted from the SBL means explaining to Spamhaus what steps were taken to stop the spam from coming. It’s a manual process with humans in the loop and can require significant business process changes for listees. (We’ve helped dozens of companies resolve SBL listings over the years, contact us if you need help.)
On the other hand, the CBL is a mostly automated list. It lists ources of mail that aren’t real mail servers sending real mail, but are sending a lot of stuff. As they describe it:

The CBL only lists IPs exhibiting characteristics which are specific to open proxies of various sorts (HTTP, socks, AnalogX, wingate, Bagle call-back proxies etc) and dedicated Spam BOTs (such as Cutwail, Rustock, Lethic, Kelihos etc) which have been abused to send spam, worms/viruses that do their own direct mail transmission, or some types of trojan-horse or “stealth” spamware, dictionary mail harvesters etc.

Because the CBL targets infected machines or open proxies, it can be automated. If a machine is sending mail that meets the criteria for infected, then it’s listed. But, as many of these infections are remediated quickly, listees can automatically remove themselves. I believe there’s also an auto expiration for IPs no longer sending.
Two different lists, two drastically different processes. There is no way a single list could effectively address both types of traffic.
We have different lists in order to address different abuse issues. Not all spam is the same, and a single list would do no one any good.
 

Related Posts

dDOS spreads to the CBL

Spamhaus has mostly mitigated the dDOS against the Spamhaus website and mailserver, but now the CBL is under attack. They have been working to get that under protection as well, but it’s taking some time.
Right now there are no public channels for delisting from the CBL. The Spamhaus Blog will be updated as things change, and I’ll try and keep things updated here as well.
UPDATE: Cloudflare talks about the scope of the attack

Read More

Fake DNSBLs

Spamhaus recently announced a few years ago that they have discovered a company that is pirating various blocklists, relabeling them and selling access to them. Not only is the company distributing the zones, they’re also running a “pay to delist” scheme whereby senders are told if they pay money, they’ll be removed from the lists.
The fake company does remove the listing from the fake zones, but does nothing to remove the IP from the original sender. This company has been caught in the past and was blocked from downloading Spamhaus hosted zones in the past, but have apparently worked around the blocks and are continuing to pirate the zone data.
It’s not clear how many customers the blocklist has, although one ESP rep told me they were seeing bounces referencing nszones.com at some typo domains.
No legitimate DNSBL charges for delisting. While I, and other people, do consult for senders listed on the major blocklists, this is not a pay for removal. What I do is act as a mediator and translator, helping senders understand what they need to do to get delisted and communicating that back to the blocklist. I work with senders to identify good, clean addresses, bad address segments and then suggest appropriate ways to comply with the blocklist requirements.

Read More

November 2015: The month in email

As we head into the last month of the year, we look back at our November adventures. I spoke twice this month, first at Message Systems Insight in Monterey (my wrap-up post is here) and then with Ken Magill at the  at the 2015 All About eMail Virtual Conference & Expo (a short follow-up here, and a longer post on filters that came out of that discussion here.). Both were fun and engaging — it’s always great to get a direct sense of what challenges are hitting people in the email world, and to help clear up myths and misconceptions about what works and doesn’t work in email marketing and delivery. I’m putting together my conference and speaking schedule for 2016 — if you know of anything interesting that should be on my radar, please add it in the comments, thanks!
In industry news, we noted a sharp uptick in CBL listings, and then posted about the explanation for the false positives. Steve wrote about an interesting new Certificate Authority (CA) called Let’s Encrypt, which looks to be a wonderful (and much-needed) alternative for certificates, and I put together some thoughts on SenderScore.
Steve and I did a few posts in parallel this month. First, Steve posted an interesting exercise in SPF debugging. Are you seeing mail from legitimate senders flagged as spam? This might be why. My investigative post was about ISP rejections, and how you can figure out where the block is occurring. In each case, you’ll get a glimpse of how we go about identifying and troubleshooting issues, even when we don’t have much to go on.
We each also wrote a bit about phishing. Steve posted a timely warning about spear phishing — malware attacks disguised as legitimate email from within your organization — and reminds all of us to be careful about attachments. With all of the more secure options for document sharing these days, it’s a lot easier to avoid the risk by maintaining a no-attachments policy in your company. And I wrote about how the Department of Defense breaking HTML links in email to help combat phishing. If your lists include military addresses (.mil), you may want to come up with a strategy for marketing to those recipients that relies less on a clickthrough call to action.
We amused ourselves a bit with a game of Deliverability Bingo, then followed up with a more serious look at the thing we hear all the time — “I’m sure they’ll unblock me if I can just explain my business model.” While an ESP abuse desk is unlikely to be swayed by this strategy, it is actually at the core of how we think about deliverability at Word to the Wise. Legitimate senders have many kinds of lists, many kinds of recipients, many kinds of marketing strategies, and many kinds of business goals. For us to help marketers craft sustainable email programs, we need to understand exactly what matters most to our clients.

Read More