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.
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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.
Mark is a professional technology writer, IT consultant and computer engineer from Dorset (England), he also founded ISPreview in 1999 and enjoys analysing the latest telecoms and broadband developments. Find me on X (Twitter), Mastodon, Facebook, BlueSky, Threads.net and Linkedin.
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About par for the course with Microslop these days.
Microsoft are just protecting their customers. Zen need to stop the abuse coming from their servers. If Microsoft are refusing to delist it means it’s ongoing.
In fairness to Microsoft, if they are getting lots of bad actors from a specific ISP’s network what are they supposed to do?
I could be wrong but based on their recent performance (e.g. updates that constantly break computers) then the problem is more likely to be at Microslop’s end. I would have thought Zen would have had the tools and diligence to identify and block abuse at their end.
This is 100% a Microslop issue. If they can’t tell what’s real and what’s spam from a major ISP (and Zen is not one known for spam unlike Microsoft themselves) then Microsoft should not be in the email business.
100% Microsoft issue yet no other ISP is having this issue? Hmmm….
@Jon Motson It is quite clear that other ISPs are or have had this issue. Read Martin from Aquiss comments below.
“IPs are not eligible for mitigation“
I don’t think it’s Zen’s fault at all. It is Microsoft who are incompet…behaving like Microsoft.
This has been an ongoing problem for me for a few years. I use my own domain and send emails from my home static IP address, this way using my own SMTP server for outgoing messages they are DMARC’d and SPF marked correctly. Was working perfectly for many years then a friend stopped getting my emails. On checking the logs of the SMTP server, it was Hotmail/Outlook rejecting them.
My IP address wasn’t black listed anywhere, but yet Microsoft blocked it. I opened a ticket, took many attempts raising tickets but eventually got to the point I could register the IP and have it whitelisted. All was then good for a month or two, then it starts getting blocked again. I get it unblocked, then it lasts a month or so and is blocked again. I just gave up in the end, my friend had a gmail address so swapped to that.
Companies that use Office 365 and email services with their own domain names don’t appear to be blocked, it’s just email going to Microsoft’s own addresses.
I use Microsoft 365 Family for a fee. My emails from Outlook do not reach their destination owing to Zen being blacklisted. Heavyweight Microsoft muscling in this way seems totally unfair to me.
I therefore had to resurrect Gmail which requires effort on my part to set up contacts as and when required. Nobody seems to accept responsibility for this fiasco.
Anyone defending Microsoft here has little to no experience dealing with Microsoft as a business.
Microslop has become the most unsufferable software provider to deal with as a business….ever.
This problem (Microsoft end) has been going on for a few weeks and unlikely not just a Zen issue. There is plenty of comments on Microsoft community forums about it all currently, which seems linked to changes they have made and now a high level of AI being used. MS seem to have also started to use proofpoint for a number of checks, which is heavy AI delivered. You literally can’t talk to anyone at MS about this.
We had a number of our MXs also impacted about 4 weeks ago, resulting in all emails to outlook, hotmail etc being deferred and rate limited for delivery. We got blanket nonsense that we were not rate-limited. You end up going around in circles.
In the end we simply had to change our MX IPs and mail delivery has returned without further issue.
Hopefully you don’t end up having to play whack-a-mole with them.
Google do the same to .uk.net and .gb.com domains. Totally blocked as spam on their end.
Reminds me of the time I once had an important official email from MS themselves appear in my spam filter.
I thought it was a scam at first because I’d expect them to whitelist themselves.
Legitimate emails being flagged as spam has been a “feature” of Hotmail for years.
I have been unable to access many azure hosted services via IPv6 since around the same time. Several of the Microsoft 365 admin centres and Anything using Azure Front Door seems to be blocked. Very annoying to say the least.
I noticed this going back a few months (back to about last September), various websites hosted on Azure don’t work connected via Zen, also login to Office 365 admin and knowledge base pages would take minutes rather than seconds.
I discovered that accessing these things with Operas built in VPN turned on, the affected sites just worked. I eventually switched to an AAISP L2TP tunnel and the problem disappeared. It wasn’t until I started getting a few calls from clients suffering similar issues that I joined the dots and realised it was specific to IPv6 on Zen…
Plusnet have exactly the same problem and don’t seem to be addressing it as I believe it has been happening since February.
As I have said before, the largest providers should not be permitted to outright block inbound mail as they are akin to a common carrier. They can flag them as spam if they want.
Since 2022 my Apple addresses (mac.com/icloud.com) have been classed as spam by most companies using Microsoft. At first I would have to tell the person over the phone to check their spam folder and 99% of the time it was there. Some people were able to get me white-listed. In the end I gave up and started sending from other addresses.
Other .mac/iCloud users have brought the issue up on the Apple support forums but the topics just get closed quickly by the admins.
Funnily enough, it’s starting to happen to the outlook.com address I also have, while my ancient MS live.co.uk address (which was meant for mailing lists and ended up a spam-magnet) always gets through OK.