Edit: This was resolved by their team, and a new reply was sent out to me:
Our system normally routes around addresses being temp blocked and should have caught that this IP was listed and removed it from service so that we could purge the bad players and repair the reputation. When we hit an IP response such as yours the action is to requeue which failed in this case. We are constantly making small adjustments to combat abuse and appear to have missed this condition in our logic.
This does not happen frequently and we apologize for the IP block based upon another sender’s behavior. This should never have impacted your messages and we have adjusted our filters to help ensure that we don’t miss similar events in the future.
Thank you for the report that helped us correct this behavior and we appreciate your patience and business!
This was just confirmed by their support team:
As mentioned by my colleague, the mail was rejected with the error “554 sorry, your IP is rejected because it was flagged as an open-relay or abuser; o seu IP foi rejeitado por estar listado como open-relay ou spammer (005) (in reply to RCPT TO command))”. On checking in detail I found that this mail was sent using an IP that belongs to a low reputation pool. As you are aware, we separate traffic based on different parameters and send it out of set of IP’s. Decision to assign a pool to a particular mail is based on its behavior/reputation. The junk pool ip can get into blacklists because of the nature of emails sent from this pool. So I would recommend you to improve the reputation of your email so that good reputation IP’s will be assigned to your emails.
This was a feature advertised and championed by them for a long time, and frankly was the feature that made their service attractive. It seems they’ve silently changed this behavior.
In this case the email simply remained listed as queued for delivery in their panel, and never was listed as rejected. The customer had to inform me of the bounce error, I wouldn’t have otherwise known. The customer is absolutely not a bad one or one that sends poorly written emails, but is actually a high quality and low volume sender.
It may be in poor taste to write this when I consider myself a competitor, but I’m also a customer and have been using their service as a fallback. I’ve always been supportive of competitors that deserved it and I’ve openly championed their service for the feature that was removed. I want to correct the record. Frankly it’s rather upsetting to receive no notice of such a dramatic change.