Since August 27th, 2025, both of our centralized mail gateways (mailgate01.uberspace.is, mailgate02.uberspace.is) seem to be on a Microsoft blocklist.
When we first reported this to Microsoft, we were told that "Nothing was detected to prevent your mail from reaching Outlook.com customers".
When we insisted that there are indeed delivery problems, Microsoft suddenly seemed to agree and declared that "these IPs are not eligible for mitigation" because they show "a pattern of irregular mail traffic", referring us to Microsoft's Smart Network Data Services (SNDS) - a debugging service that we are subscribed to for years, that we regularly check, and that continues to tell us that "All of the specified IPs have normal status".
We are also subscribed to Microsoft's Junk Mail Reporting Program (JMRP) so we directly receive complaints. Those complaints are usually bogus and often refer to mail that's clearly not spam at all, but Microsoft customers marked them as spam nevertheless. SNDS itself declares a complaint rate of <0.1% for mails coming from our mail gateways.
Generally, if there's any kind of spam incident that we get aware of by any means (our own queue monitoring, external abuse reports, our IPs appearing on a well-known blocklist, ...) we immediately investigate and block mail accounts that happen to send spam (e.g. because their password got compromised). No other well-known blocklist reports our mail gateways as problematic.
We continue to communicate with Microsoft's "Outlook.com Deliverability Support Team" and try to have our mail gateways removed from their blocklist. If and when they do is not under our control.