Microsoft Domain Blacklist Causes Email Problems for UK ISP Zen Internet

Customers of broadband ISP Zen Internet appear to have been experiencing difficulties sending emails to Microsoft’s popular email services (Outlook / Hotmail etc.) since last week, which occurred after the software giant “blacklisted some domain servers hosted in our network“.

The issue appears to have started on 10th March 2026 and is currently still ongoing over a week later. According to the related Service Status Notice, Zen’s Information Security team has been liaising with Microsoft in an attempt to achieve a full delisting, but as of yesterday the software giant was continuing to inform Zen that their “IPs are not eligible for mitigation“.

In fairness, it’s important to understand that issues like this do crop up from time to time between ISPs and major email providers, which often occurs when a sizeable volume of abuse is detected (e.g. email spam/scams/malware etc.). Such abuse may occur when some customers or their devices are hijacked to send masses of junk email and malware, but there can be other reasons too.

The Service Status Notice doesn’t elaborate on what reasons Microsoft has given for blacklisting some of Zen’s domains, but we have asked Zen to comment and will report back later. Credits to Tony for the news tip.