Seemingly out of nowhere after literally over a decade of using Spamhaus with no issues, people were having issues sending to us. Their emails were getting flagged as spam – or at least so it would appear to them – they were receiving rejection notices saying that our email server rejected their message as spam. When I combed through our server logs, I noticed the error message actually seemed to indicate that the spamhaus service was unavailable and that’s why it was rejecting mail:
Remote Server returned '550 5.7.350 Remote server returned message detected as spam -> 554 5.7.1 Service unavailable; Client host [18.104.22.168] blocked using sbl.spamhaus.org'
You’ll notice it says “detected as spam” but it also does specifically say: “service unavailable.” So what gives?
Well, thankfully, the geniuses over at MXRoute.com recently posted an article and have asked for help spreading awareness of an issue, so that’s precisely what I’m here to do. Spamhaus has made the horrible decision to no longer accept DNS requests from the most common DNS servers. In other words, if your mail server is configured to use something like Google’s 22.214.171.124, or Cloudflare’s 126.96.36.199, there’s a good chance that your request is simply going to get rejected. This, in turn, will cause your server to receive a “service unavailable” message like the one above, which will force it to send a rejection notice to the original sender.
Why was I impacted by this “seemingly out of nowhere?” Because I had recently switched my DNS settings to use 188.8.131.52. Surprise!
You can fix this quickly by switching, painful as it may seem, to your local ISP’s normal DNS entries, or there are ways you can see if you can have your server’s IP address added to a sort of approved list for low volume senders.