ISP mail servers blocked by Microsoft

I’ve not been able to send email to outlook.com addresses for more than a week now. My ISP confirmed that they have been placed on a block list by Microsoft that prevents email being sent to outlook servers. Has anyone else had this problem? Below is the response from the outlook server.
outlook-com.olc.protection.outlook.com [52.101.194.1] SMTP error from remote mail server after pipelined end of data:
550 5.7.1 Unfortunately, messages from [212.23.1.20] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150).

Hi

Its a fairly well known issue with Zen, have a look here: https://servicealerts.zen.co.uk/alert/11226/
and
https://www.ispreview.co.uk/index.php/2026/03/micros...

Andrew

Whilst it doesn't solve your immediate issue, now would be a good time to start moving your mail away from Zen. There are many more reliable options, including free ones.

It happens occasionally with shared servers. I deal with a number of local authorities and email sometimes bounces on spam score because others on the server are misusing it. It usually corrects itself after a few days.

On a related issue, I did get a bounce last week on content. I ran the wording through a spam checker and found that 'get' and 'opportunity' shouldn't be used in emails.

On going problem for me as well, I'm with Unchained, exactly the same issue. I spent half day with emails back and forth with Microsoft support and eventually was added to a whitelist. All was okay for a couple of months then it got blocked again, so I had to go through the process all again. Same pattern, okay for a few months then blocked again.

Why Microsoft are so dumb over this I don't know, its only them I've ever had this issue with. My IP appears on no block/black lists, and the number of emails I was sending to them was a couple a day if that to the same recipient, and DMARC and SPF all set correctly, hardly a sign of a spammer.

You can try sending an email to [email protected], or using the portal at https://sender.office.com/, this may tell you that you are not blocked by Office 365, and then provides a link to delist from hotmail/outlook domains. Any delist however doesn't seem to stick for long.

Upshot was I've had two friends quit using Hotmail/Outlook, they had found emails from a few different sources would never show up, so when I explained why that was likely, and that they are dropped and they would never know that was the case, i.e. not even in the SPAM folder, they didn't want to risk missing emails again so happy to move.

I've found though that companies using Office 365 for email, I'm never blocked from there, only from sending emails directly to @outlook.com @live.com and @hotmail.com.

Edited by E300 (Tue 31-Mar-26 14:04:19)

Have ran email servers for many years, and the single biggest headache is Microsoft, their filtering service is particularly aggressive, as you said, the whitelist seems to also only be temporary.

I tried the portal address that you quoted - https://sender.office.com/, but the following comma is treated as part of the link so got a "Server Error in '/' Application."

https://sender.office.com/ worked fine.

I suspect that the the reason that you have to repeat the delist may be down to not having a static ip address. I see that the portal asks for this as well as the email address, When the lease on your current ip address expires and a fresh one is issued presumably the delist then fails.

Quite why Microsoft do this is a mystery, as if the delisted address is verified by them as part of the process it should normally be fine with a new ip address - unless of course the email address is actually the one doing the spamming.

But then surely their filters would pick that up again and react accordingly? Or is that logic too simple for them?

In reply to a post by RussellR:

I suspect that the the reason that you have to repeat the delist may be down to not having a static ip address.

I have a static IP address, so its not that.

The advantage to Microsoft is that by blacklisting other providers of e-mail services it encourages more people to use its own services.

In reply to a post by ajseeds:

The advantage to Microsoft is that by blacklisting other providers of e-mail services it encourages more people to use its own services.

??? Having experienced these problems sending to Outlook, the last thing I would do is cause problems for myself and others by taking a service from them. I would be more likely to suggest that existing user who are not getting my emails could resolve the problem by leaving Outlook