But abusix with spamassasin, is good but still got a lot of spam passing, because SA don't support signature base abusix list.

@girish BTW is there a way to manually release "blocked" messages or let them through?

black.junkemailfilter.com;bl.mailspike.net;all.spamrats.com;zen.spamhaus.org

interesting, I never eared about mailspike before
mainly I use [dbl and zen]spamhaus + barracudacentral.org

Also on wikipedia they have a great listing with nice description:
https://en.wikipedia.org/wiki/Comparison_of_DNS_blacklists

Thanks to @d19dotca and the Cloudron Team!

Since many visit this thread (it's even linked in the documentation now too!) for the list of the various RBLs and experience with them, reviewing them, etc... I wanted to add one more to the list which I've been testing out for a little bit and so far seems great, blocking spam from bad IPs which even hours later still isn't on some of the other popular blacklists when I've been checking manually to verify things.

Abusix is a premium service, however they do have a free tier which offers a rather large 5,000 queries per day - and I suspect most of us are not close to that amount in a single day, many likely not even over the course of a week - effectively meaning we can get premium-level spam filtering for free. They have several different lists they manage, but the recommended one to use is their combined.mail.abusix.zone zone which checks three separate lists of theirs out of the several. It is their "recommended" one for production servers offering a good balance of more checks and performance using one single lookup zone without being too overbearing as to include false-positives, this way it greatly limits any false-positives (of which I've seen zero so far!).

The only downside is a very minor cosmetic issue in Cloudron with it as the Abusix list is something like <UUID>.combined.mail.abusix.zone since it's premium so it's a unique URL to every user, and as such it's a very long URL due to the UUID which means some of the log entries in Cloudron's UI for denied messages get pretty long looking. I may file a feature request later for us to perhaps try naming our zones how owe want them to so we can avoid really long named ones in the logs, but overall it's just a cosmetic issue and nothing else.

So just to summarize, the ones I'm using with great success so far are the following:

<UUID>.combined.mail.abusix.zone
zen.spamhaus.org
bl.mailspike.net

The zone I've settled on and most recently been using...

black.junkemailfilter.com;bl.mailspike.net;all.spamrats.com;zen.spamhaus.org

To give a bit of context to each of them...

• black.junkemailfilter.com was one I added the other day because of a clearly-spam message getting through and I had seen it in the "just now" timeframe so quickly checked mxtoolbox.com to see which DNSBL had it listed, and four of them did. This was one of them. The other was Barracuda which requires registration that I didn't want to try yet, and the other two were UCEPROTECT-2 and UCEPROTECT-3 which I didn't want to use (see side note at bottom). It’s caught a fair bit of spam the others didn’t catch earlier. This is known as "JMF-Black" on the Intra2Net list and has zero false-positives.

• bl.mailspike.net is one that seems very useful and is very accurate, has helped block a lot that Spamhaus Zen didn't catch earlier (before I had the zen.spamhaus.org earlier in the list meaning it should be checked first so we'd know if anything else blocked it that Spamhaus Zen didn't have yet). This blocked a good amount of spam. Zero false-positives.

• all.spamrats.com is also an excellent one and I'd say blocked as many as Spamhaus Zen did, it was catching spam frequently. Earlier I was using the noptr.spamrats.com which worked very well too but later learned of the broader all.spamrats.com and started using that yesterday with continued success and zero false-positives still.

• zen.spamhaus.org needs no introduction, it's probably the most popular DNSBL ever created. Much like the others, it's highly accurate and hasn't been seen to have any false-positives.

For all of the above and more, I'd recommend checking out the Intra2Net service which monitors the accuracy of the various DNSBLs. My recommendation is to stick to ones if blocking from the MTA level that has a 0% or at least no higher than a 0.05% chance of false-positives as they'd be considered safe. All the other DNSBLs that are more aggressive should really just go a step down to the SpamAssassin level for scoring metrics there (which I later did too for the URIBLs instead and they've been great so far too). I did have two false-positives in very early tests with the dnsbl.sorbs.net one and also with bl.spamcop.net one. I'd suggest avoiding those for denying connections but can be used in SpamAssassin instead.

I hope the above is a good test report for people, and others will hopefully find this helpful. Certainly there is no one-size-fits-all approach here, and I'd argue that none of these should even be enabled by default, however I believe them all to be "safe" based on my own experience and was glad to see the spam cut down further than it was prior to adding in the additional DNSBLs. What works for me may not work for you of course, depends likely on a lot of different factors, so "your mileage may vary" as they say. I've been watching the logs like a hawk all week and been checking every single "denied" entry and happy to report no false-positives in my testing so far over the last few days using the four listed above.

Here’s a quick screenshot:

Side note regarding UCEPROTECT DNSBLs: I strongly discourage use of the UCEPROTECT-* lists except possibly UCEPROTECT-1, because the level 2 and 3 seem to just blacklist large IP ranges that affect entire providers such as DigitalOcean, OVH, and more and basically demand fees for "express delisting" which doesn't even guarantee anything as it can be re-listed the next day. I question the ethics of that particular DNSBL provider as they seem to "extort" money from large network providers, and there is also this article I found that pretty much strips them apart line by line and explains why they may not be good to trust or use. My advice is to stay away from the UCEPROTECT DNSBLs based on the above plus they'd surely have a fairly high false-positive rate (you can see from the link above that the UCEPROTECT-3 has a whopping 17% inaccuracy rate.

@girish - Hopefully the above report will be useful for you and @nebulon when discussing some of the mail changes that may be coming in 6.3 there.

Update - March 26, 2021: Out of thousands of emails over the last week, I've only found two false-positives (thankfully non-critical emails, one was a Snapchat newsletter for example). That is a very impressive result to me and my users and I'm pleased with that as that seems to be within the reasonable threshold when weighing the pros and cons.

With that said, I have started a second test which involves removing one of the DNSBLs which made the false-positive result, and then added instead to the SpamAssassin side of things to at least help with identifying spam better to avoid the inbox. While this has led to more spam processing on my server, it seems to still be working well to achieving the ultimate goal of keeping spam messages out of my users inboxes. Here is my current DNSBL list zones in effect: zen.spamhaus.org;bl.mailspike.net;noptr.spamrats.com (notice I removed the black.junkemailfilter.com and changed from all.spamrats.com back to noptr.spamrats.com)

So far the results are good. This however means unfortunately some of my clients who have mailing lists on the server that forward to their personal accounts elsewhere are receiving a bit more spam again until the new feature request is implemented to prevent external spam messages from being sent.

Depending on the results of the above tests, I may either stick to the current implementation or go back to how it was last week.

In my case as an example, I've had a lot of email denied now at the MTA level by adding in the extra DNSBLs, and they've worked perfectly with zero false-positives so far in nearly 48 hours of running it on a very actively used mail server. So I see that as a big win for me and my clients. And of course I can always reconsider if I start seeing too many false-positives (or heck, even one false-positives for that matter - which I've yet to see so far in my testing), and that's kind of another example of the point I'm making here too... that we need options so we can enable and disable at will. In other words, none of what I'm asking for or proposing should be enabled by default, and none of it should be hard-coded as it currently is - we should be exposing this to users so they can make the decisions that best suits their own needs.

I know you seem worried about "complexity to cloudron" which is a fair concern always on how to keep Cloudron user-friendly, however I don't quite share that one this time because the data is already in Cloudron (it already uses Spamhaus Zen for blocking at MTA level and as we've seen is easily configurable already - it just doesn't survive restarts the way we can manually do it now), so the existing functionality just needs to be exposed in the UI. It's really no different than how they had recently exposed the SpamAssassin configuration too. It just needs a new little line on the Mail page with the other settings for message sized and such, a toggle and a box to list new DNSBLs. Of course it's more complicated than that in the backend, but from a user perspective this should not add any additional complexities. And I'd assume even in the backend once it's done it's done and shouldn't really need to be maintained at all.

All to say... we should have options available to us to benefit all of us uniquely as running a mail server is a complex task where it is not feasible to use a one-size-fits-all approach, IMO.

That is an interesting approach, and I'll consider it.

I know that is not the best solution out there.
But you can automate it, with API and Sieve is an open protocol.

But is a start for now, and yes having a mail filter also for mail fwd is a good option, and if you don't need it you should just be able to disable SpamAssassin from the server.

I really don't like to w8st resources

spam will never be 100% blocked and there will always be false-positives too

Yes, I totally agree with that.

By big question here is how much is useful to add feature and complexity to cloudron, when there are solutions that are specifically built for that.

About mailspike and Spamhaus, they both have SpamAssassin module, and if cloudron provides the possibility to disable SA, I think is a grate idea to have them.
But not as simple DNSBL, but as SA module.

For spamrats idk, having to many DNSBL or SpamAssassin module will slow down the server a lot, remember for every incoming email, you need to call unbound that check his cache and if doesn't have the record call the DNSBL.
This happens for every email, and every external check you have, and maybe they are slow at that moment and the request take more time than usual, ... and so on
Resources usage need to be taken into count.

For anyone who doesn't want Mailspike actually denying connections though, it should also work great in SpamAssassin as they already gave all the needed headers for it too so at least it'll help in identifying spam better too, using the text below:

header RCVD_IN_MSPIKE_BL eval:check_rbl('mspike-lastexternal', 'bl.mailspike.net.')
tflags RCVD_IN_MSPIKE_BL net
score RCVD_IN_MSPIKE_BL 3.5
header RCVD_IN_MSPIKE_WL eval:check_rbl('mspike-lastexternal', 'wl.mailspike.net.')
tflags RCVD_IN_MSPIKE_WL net
score RCVD_IN_MSPIKE_WL -2.1

• The blacklists still are working perfectly with no false-positives since I changed it to the one above. I also added one late last night (because I saw a few getting through that were clear spam again) the noptr.spamrats.com which according to blacklist checks the spammy servers were listed on it. So now it reads as follows: zen.spamhaus.org;bl.mailspike.net;noptr.spamrats.com;bl.0spam.org - So on the side of denying more spam connections while not having any false-positives, this seems like a huge win. Notice the many emails blocked from sources other than Spamhaus Zen, and I've confirmed none of them are false-positives.

• On the other side of things... I noticed a clearly-spam message getting past still though and reported it in the other thread.

A further update to my testing of manually adding in the DNBSLs:

• I have settled for now on the following zone, which so far has worked perfectly with no false-positives today with these three DNSBLs set in Haraka (but I'm very carefully monitoring this and may make further tweaks if needed): zen.spamhaus.org;bl.mailspike.net;bl.0spam.org
• Two previous tests were run with zen.spamhaus.org;dnsbl.sorbs.net but the SORBS quickly caught a false-positive within minutes, so I removed SORBS in favour of zen.spamhaus.org;bl.spamcop.net and while that was much better it still got one false-positive after a few hours so since this is all being tested I opted to try the above and current list of zen.spamhaus.org;bl.mailspike.net;bl.0spam.org which so far has worked perfectly with no false-positives, but am still testing and watching the "denied" mail server logs carefully for any false-positives.

usually forwards happen before a sieve filter, so unless you can only fwd things from a specific folder like inbox, it's going to send everything.

will not resolve the issue and make the setup harder for newbies

I don't think I agree with that. Nothing will "resolve the issue" of spam itself (if that's what you meant by "the issue"), spam will never be 100% blocked and there will always be false-positives too. The goal is simply to reduce the level of spam and reducing the level of false-positives (or at least keeping it at an acceptable level), and that's where the extra customization comes into play.

I also don't think it will make it harder for "newbies" at all, because the out-of-the-box Cloudron setup would not change (at least I don't envision it would). Having the ability to set extra DNSBL checks for denying messages before they get processed shouldn't make anything harder for anyone - I should be able to setup a new Cloudron instance just as easily as I can today. Nothing should change there. Only the option to add new DNSBL checks to deny messages for example would be added as a completely optional feature to enable - it'd basically only be touched by "power users" and actual mail administrators who are comfortable making those tweaks and already looking to make such changes in the first place.

I agree though that there's plenty of different ways to improve spam filtering and this is just one of many possible ways that I hope to see (and many others from the community too judging by how many mail improvements / feature requests exist in the Cloudron forum).

For the fwd issue use a sieve forward from imbox this should prevent email marked as spam to been sent out

That is an interesting approach, and I'll consider it. My first thought though is... while it may technically be a valid workaround, in my case I don't think this option is feasible though as I have too many accounts to do this for. I have roughly 20+ recipients on my server who only are setup for mailing lists to forward to their own personal email accounts on common domains (no mailboxes on Cloudron). This workaround means I'd need to setup about 20+ mailboxes, not only that but also set them all up consistently and accurately. This leaves a lot of room for human error in my case and a lot of overhead if I ever wanted to make a quick tweak and keep it consistent across them all. If I only had a few, that'd be no problem, but I think I have too many for that to be feasible in my case, unfortunately. I appreciate the thought there though, I hadn't really considered that as a possible workaround before.

I'm sure that with better setup of DNSBL, URIBL,DCC, and SURBL will be better, but will not resolve the issue and make the setup harder for newbies.

For the fwd issue use a sieve forward from imbox this should prevent email marked as spam to been sent out.

Sorry if miss some point, I'm in paternity leaving so sleep is not a thing. (not for a baby. for now just an adorable husky that doesn't understand that he can sleep at night)

That list essentially monitors many DNSBLs for effectiveness and inaccuracies too (false-positives) using their own network for running the tests on. I find it quite interesting and stumbled into it today again, and I remember seeing it many years ago too. It's always up-to-date data which is interesting.

I think it was both SpamAssassin and SpamCop I used on my mail server before Cloudron, so I've set it accordingly now. It now reads zen.spamhaus.org;bl.spamcop.net for the zone.