All of a sudden, outbound mail for all users across all domains is failing with errors like this:
Service unavailable; Sender address [[email@example.com] (mailto:firstname.lastname@example.org)] blocked using [dbl.spamhaus.org](http://dbl.spamhaus.org); Error: open resolver; https://www.spamhaus.org/returnc/pub/184.108.40.206
That IP address (and others that are reported in similar errors) belongs to CloudFlare, which I’m not aware of using at all. It’s not the IP of my mail server, nor its DNS server (I’m not using MIAB as my DNS server, and the mail server itself is using my hosting provider’s DNS (Scaleway)), nor the client’s IP. When I visit the SpamHaus info pages for any of the domains or IPs, it reports that there are no problems, nor do they appear on any blacklists that I can find.
Curiously, this only seems to be applied to external mail clients. Webmail on the same server works fine.
I updated all packages, re-ran the MIAB installer and rebooted, but nothing changed.
Anyone have any bright ideas what the problem might be and how to solve it?