Our current plague of revolving .top and .click spam email domains
Email spam is somewhat like the weather, and much like the weather I don't talk about it much any more. However, every so often something unusually unpleasant happens (in both of them). Our current irritation in spam weather is what I suspect is one particular spammer that operates using a rapidly changing flux of spam domains in .top, .click, and on some days .us, using a distinctive (but not really machine matchable) pattern of tagged envelope senders.
The typical pattern of envelope senders are ones that look like this:
This '<phrase>-<user>=<domain>@<random>.(top click us)' envelope sender is quite human recognizable and is clearly tagged, but it's not all that easily matched without false positives. The tagged envelope sender is less useful than it looks, because none of these domains actually accept email.
The spammer is fast moving at changing both sending domains and sending IPs. Their domains and IPs tend to wind up listed by people like Spamhaus within an hour or three, but by then they've moved on. They don't seem to reuse domain names very much (or very fast, when they do reuse them), but in a spot check they did reuse IP addresses over the past couple of days, perhaps as they fall out of the SBLCSS and similar DNS blocklists. Possibly the spammer reuses domain names less often due to them expiring from DNS blocklists more slowly than IPs.
(In fact now that I'm looking at this seriously, this spammer appears to be only using three /24s for the past week or so.)
Unfortunately our current anti-spam software (rspamd) doesn't immediately recognize this mail as spam (although once things are DNSBL listed it can do better). GMail is wise to their tricks, of course, and so email from this spammer to people here who forward their email to GMail is rejected by GMail at SMTP time, giving us bounces that pile up in our queues as we try to deliver them to the spammer (who is, as mentioned, not taking email). The messages have valid DKIM signatures and even pass SPF checks (to my amusement the spammer thoughtfully lists the domain's sending IP in their SPF record).
(GMail typically rejects the email with messages about the reputation of the sending domain being too low, but as I've seen with Message-IDs, GMail's rejection messages aren't necessarily anywhere near the whole truth.)
In a spot check of these domains, DNS service is being provided by Cloudflare, perhaps via some free plan or perhaps as part of the registrar's offering. WHOIS for recent domains lists the registrar as NameSilo LLC (although the domains might have been obtained through a reseller), who appear to offer very low up front costs for registering domains in some of these TLDs. Still, the churn in domain names suggests to me that the spammer probably isn't paying for them in one way or another.
The side effects of this particular spammer are sufficiently annoying that I may take some specific steps to deal with them. While there are a bunch of clever, complicated options, it's possible that quite brute force ones would be sufficient.
(Mostly I'm irritated that people are letting them get away with going through so many domains. Domain registration is supposed to cost money, and domains aren't supposed to be expendable things for spammers. Yet here we are, with 'fast flux' domain names.)