Web Hosting Canada Broken

Finished Scheduled on 2022-05-03 15:10:00 Estimated finish 2022-05-05 18:14:00
Recipients who use the hosting service Web Hosting Canada (whc.ca) are unable to receive mail today. Their servers are all returning this error:

550-"JunkMail rejected - mail-108-mta167.mxroute.com []:35483 is in an RBL: Error: open resolver; https://www.spamhaus.org/returnc/pub/")

At first glance it sounds like they're saying that our IPs are listed at SpamHaus but that is indeed not the case. In fact, it is a problem and misconfiguration with their servers which is causing one of their IPs,, to fail their own RBL checks on their side.

We will try to reach out to them, but it is uncertain whether or not we will reach anyone who understands the problem. In the meantime, recipients using that service may simply be unable to receive mail.

Inevitably someone always misinterprets an announcement like this, so this is important: This is not a fault with our service, this is not a fault with how we communicate with their service. This is entirely a third party fault, the only connection to us is that a few of you are sending emails to people who use their service which is, today, broken.

Technical details, for those who care:

WHC is running an open resolver which they use internally to cache queries made to SpamHaus. The resolver is supposed to be limited to their servers, not open. They left it open, and now SpamHaus is refusing queries from their server. As a result, all of their servers now think that every query against SpamHaus is returning true, and that every connecting server is blacklisted.
Related servers / services:
Date Action
2022-05-03 15:19:00 Response from their team:

"We do apologize for the inconvenience caused to you. We are aware of this issue and our admins are already working on the same. This should be get resolved ASAP. Meanwhile, your patience is highly appreciated."