: Email flow slowed or stopped by mysterious forces at Microsoft
Microsoft spent last week rejecting emails to Outlook recipients after what appears to be either a fault or overzealous blocking rules, a situation a source described as "carnage."
The problem affects certain IP addresses, whose emails are rejected due to falling foul of reputation rules or appearing on a block list.
A Register reader told us, "At the back end of January we noticed a sudden spike in customers static IPs being rejected by only Microsoft Outlook free / personal accounts."
The message returned was a 550, telling customers to contact their Internet Service Provider (ISP) "since part of their network is on our block list."
A block list is a good thing. It helps stem the flow of spam from networks or addresses associated with junk email. However, the confusing thing for our reader is that his company was not on Microsoft's naughty step for email. A look at Microsoft's Smart Network Data Service (SNDS) showed no issues with the IP.
"We're also a member of their JMRP (Junk Mail Reporting Program)," our reader added, "which is intended to inform us when people are reporting spam sent from our IPs - except, we never get any reports."
The problem worsened in February. On Microsoft's support forums, users began to complain about similar issues as the IP net presumably widened. One wrote: "We are currently experiencing a critical and recurring email delivery issue affecting recipients at outlook.com, live.com, hotmail.com, and msn.com," and provided a copy of an error that suggested the mail server has been "temporarily rate limited due to IP reputation."
The user drily noted, "Although the error indicates rate limiting, in practice no emails are being delivered."
A large number of users, ranging from the administrator of a server sending automated notifications on behalf of Estonian Public Libraries to an email provider for healthcare professionals, chimed in to confirm they too were having delivery problems and Microsoft support was not helpful.
Our reader told us, "We've seen customers struggling to send invoices, order delivery notifications, authentication codes - all sorts, which have been perfectly acceptable to Microsoft / Outlook for many years - now rejected, or blocked."
They pointed out that when a user sees a 550 error, they don't always realize the receiving mail server is refusing the message, instead assuming that their own ISP is blocking their outgoing email.
"Customers rarely read or understand [delivery service notifications] - they jump to blaming the ISP or sender, and then head off to find someone else."
Unsurprisingly, our reader spoke on condition of anonymity - nobody wants to be the ISP that has to say, "Yeah, we can deliver your email anywhere but Outlook.com" to customers.
We asked Microsoft to comment, but other than acknowledging our questions, the company did not respond further.
Anything that reduces the amount of junk cluttering up inboxes is to be applauded, however, if a vendor makes an error, customers need a rapid and transparent process for resolution.
Every failed delivery of an invoice or receipt due to overzealous or misapplied rules can chip away at a business's reputation, through no fault of the owner. ®
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Comments
By VladVladikoff 2026-03-0419:594 reply This has plagued us for years. We send quite a lot of transactional email (about 150k emails per day), and there have been several times where Microsoft blocked our server. Usually it is because Microsoft has banned an entire netblock, that our server just happens to be sitting in. I have seen them do this to IPs fro Hetzner, Linode, Amazon AWS (SES), etc. And yeah we've signed up for their junk mail reporting service, and we have all our DNS records dialed in perfectly.
I even went as far as signing up for Azure, in the hopes that if I sent from a Microsoft IP it might not get blocked. But I didn't make it very far, every step of the way was like watching paint dry while the interface loaded or did something. Once I finally got the thing set up in order to send mail, the API was so molasses slow that it couldn't handle our mail throughput. Meaning it would take about 30 seconds to send each transactional email because of how slow their API is. Well that's only 2880 emails per day, that is not a reasonable send rate at all.
I have even lost customers over this mess, it's really hard to explain to them that they can't receive our email because of their provider and not us. Especially when Microsoft has the audacity to return: 250 OK Email Queued (but then not deliver it anyway!)
If anyone has any solutions to this mess I am all ears!
By tokyobreakfast 2026-03-0420:093 reply It's almost certainly because your customers are reporting your emails as spam by moving it into the junk folder which is training their systems.
Once enough of your customers do this to cross a certain threshold, you are identified as an undesirable sender and QED.
It happens at scale.
There's like a middle scale where you're not big enough that Microsoft will go out of their way to whitelist you, but you're big enough that your "send to junk" rate is just high in terms of absolute numbers.
It's certainly not a ratio, it must be based on absolute numbers because I've seen it too many times across too many companies, and the only ones that get away with it are extremely low volume.
Once you have 1,000,000 mails, even a 0.1% mark as spam rate is 1,000 emails. - and some people treat mark as spam as their delete button, certainly more than 0.1% of people. Don't ask me why.
EDIT: on inspection; it's worth noting the mechanism is even more insidious than "people mark you as spam". Microsoft also weighs delete-without-opening as a negative signal. So if you're sending transactional mail (receipts, shipping notifications, invoices) and your users get exactly what they wanted, feel satisfied, and bin it without opening. You've just taken a reputation hit for doing your job correctly. The senders most at risk aren't the ones sending rubbish.
EDIT2; theres a reply to me that I can’t reply to because its [dead]; though the point is valid so I vouched. To them I say: I agree. But you probably want your receipt, and thats the example I gave (for a reason).
By LorenPechtel 2026-03-0422:17 Delete without opening shouldn't be treated as negative. I generally keep the stuff on your list, but I almost never open the it's-your-turn e-mails--the title contains everything I need to know. And, likewise, Meetup notifications--I dump most of them unread as the title is enough to tell me I'm not interested in that event.
By justinclift 2026-03-0423:241 reply > ... some people treat mark as spam as their delete button, certainly more than 0.1% of people. Don't ask me why.
If a business sends mail that I've not previously indicated I'm ok with them sending (newsletters out of the blue, unasked for "summary of stuff you've missed" emails, etc), then it's spam and will be treated as such.
By Bombthecat 2026-03-0515:23 Yap. Five years or more I would have just deleted it. Now? Spam, block. Done
By GCUMstlyHarmls 2026-03-051:46 > and some people treat mark as spam as their delete button, certainly more than 0.1% of people. Don't ask me why.
The delete button is for saving important stuff, not spam.
https://old.reddit.com/r/sysadmin/comments/jndew2/users_who_...
https://old.reddit.com/r/talesfromtechsupport/comments/1sac4...
https://old.reddit.com/r/talesfromtechsupport/comments/1eun2...
I recall seeing this was some kind of learned behavior from lotus notes, deleted items wouldn't count in your quota, so users would keep stuff in there.
By andrewmcwatters 2026-03-0421:57 Because everyone and their mother sends spam. It doesn't take a genius to figure out why. No, I don't want emails about the new product you're releasing. No I don't want your newsletter.
I have to actively fight to keep my inbox empty, and free of crap. And while I'm ranting, notifications I actually want on my phone are co-opted by advertisements, which Apple and Google should actively prevent, but they won't because they use push notifications to commingle advertising to you with important, sometimes time-sensitive notifications.
Everyone thinks they have something worth sharing. Over three nines of the time, people don't. I will happily be apart of the 0.1% who sends your crap straight to the spam filter for Gmail to train.
It's spam. It's almost all spam. Even the transactional stuff for logging in, I don't want. Just use an email, password, and TOTP. Stop sending me emails.
Stop it.
By adrian_b 2026-03-0420:49 I do not think that is true.
Once when this happened to me a couple of years ago, it was the opposite.
My e-mails were put by default by Microsoft as spam into the junk folder, without the customer knowing anything about this.
After I succeeded to notify him about this, he searched there the e-mails and marked them as "not spam", and then he received my following e-mails.
So initially the customer did nothing and was not aware that some of the e-mails sent to him are classified as spam, and he had to do active efforts to override this default action by Microsoft.
There was absolutely nothing suspicious about the e-mail messages classified as spam in their content, their only fault was not coming from one of the few major e-mail providers.
By bluecalm 2026-03-0422:16 Almost certainly not. Microsoft was the only email provider we had problems with a few years back. We only sent emails to people who paid us several hundred USD to send that email. We didn't sent anything else - no follow ups, marketing, announcements, nothing.
By linsomniac 2026-03-0516:29 You probably want to sign up for SNDS: https://sendersupport.olc.protection.outlook.com/snds/faq
That should allow you to be more proactive about users reporting your messages as spam, either intentionally or unintentionally.
FWIW: We've been sending Microsoft properties e-mails for over a decade, fairly small scale (maybe 5-20K unique recipients at MS properties in a month), and every 2-4 years we have to submit our IP to their "whitelist me" site and then we're golden again.
This time was different, when we submitted our IP to the whitelist site it said "Nothing is blocking your ability to send to us". They did end up responding to our whitelist request a week later asking if we were good or still needed help, which is a first.
By wolvoleo 2026-03-062:51 They also block people who send very little mail. If you only send one or two per month to their consumer domains you don't build enough 'reputation'.
I know for a fact my mail server never sent any spam because I logged everything. It was only me using it. But every few months I got banned from sending to live.com. I have one friend there. They didn't mark anything as spam. We just sent personal emails.
There's a form where you can unban it but it kept happening. In the end I stopped running my own, it was so much hassle.
By Bombthecat 2026-03-0515:07 Send paper to those customers:)
By zelphirkalt 2026-03-0418:201 reply It is my experience, that Outlook is not a reliable e-mail service. Sometimes e-mails are not delivered, or only delivered hours later. When they are delivered, even as a paying customer, they are downloaded so slowly, that I had to wait 10 minutes to get all my e-mails, while my 1 EUR per month Posteo provider delivers in seconds.
My impression is, that the only reason one would want to have MS as a mail provider is, that they are entrenched in the e-mail provider reputation and delivery game. Other than that, it seems to be an all around bad service. Not even talking about the mail client itself.
By SoftTalker 2026-03-0419:321 reply The big reason is enterprises buy into O365 and running their email through Outlook instead of on-prem or at another provider is part of that. For the same reason they use Teams over Zoom or Slack or other alternatives.
Exactly. Nobody chooses MS for quality, and those who do choose it are never those who suffer the most for its own decisions.
By wolvoleo 2026-03-063:17 Yup. It's really the cheapest not the best. And Microsoft sell directly to our C suite who have white glove IT support that do everything for them so they never see any issues.
The old saying of "Microsoft is better at talking to your boss than you are" is certainly true though. Unfortunately.
By Ensorceled 2026-03-0412:393 reply My clients have been experiencing this forever; the logs SAY "temporarily rate limited due to IP reputation." but really the emails are never going to get delivered. I have to get MailChimp or Mailgun to rotate the IPs.
It looks like all it takes is one person to mark your email as spam, even by accident. Note that these are mailing lists which they signed up for in MailChimp case OR transactional emails in the Mailgun case.
It's only hotmail/outlook that we constantly have this issue with, Google etc. are all fine.
Often these "spam" reports by end users are just accidental clicks as well. Many of the abuse reports we get are like an email from someone's Mum and visibly legitimate. At other times there are users who use the Report Spam function as a kind of inbox management tool - a way of moving mail away so they don't have to see it because Trash or Delete or whatever is just further away from their pointer.
By pluralmonad 2026-03-0412:581 reply I tell my friends and family to never click unsubscribe links, unless they had proactively subscribed. Buying something from a company that requires an email does not count. unsolicited marketing emails are spam and should be treated as such. Double so if that company sends marketing emails disguised behind support@company.com.
By iamacyborg 2026-03-0413:292 reply > Double so if that company sends marketing emails disguised behind support@company.com
That’s typically not a disguise but a clear means of indicating that you can reply to the email
By pluralmonad 2026-03-0414:561 reply No, sending marketing from support emails is almost certainly trying to game spam filters. Marketing@company.com would work for the allow replies purpose.
By iamacyborg 2026-03-0416:291 reply > sending marketing from support emails is almost certainly trying to game spam filters
That is not how spam filters work.
By pluralmonad 2026-03-0418:44 If I've interacted with a specific email address, like support@company.com, my email provider will put them in my inbox.
By ycombinatrix 2026-03-0417:45 How is it not a disguise? It means you can't block marketing emails without also blocking the legitimate support emails.
"Report spam" is quicker and easier than "unsubscribe".
Gmail added a popup asking the user if they want to unsubscribe when flagging a newsletter with the appropriate unsubscribe headers, so it must be common enough to warrant Gmail developer attention.
By vintermann 2026-03-0510:441 reply Thing is, unsubscribe links are often an "inform spammers that this mail address is in use" link. Even the ones Gmail offers up.
If I didn't click their button to subscribe, I'm not clicking their button to unsubscribe. Who's to say they won't just "sign me up" again after a while? In fact I know several large US corporations which routinely sign you up for notifications again after a few months.
If you didn't click the button, then of course, you don't send the unsubscribe message.
If you did subscribe to a newsletter and no longer want to receive it (which is the majority of these cases), then the unsubscribe action is the logical thing to do.
By Ensorceled 2026-03-0512:18 > If you did subscribe to a newsletter and no longer want to receive it (which is the majority of these cases), then the unsubscribe action is the logical thing to do.
Not the second time :-)
By Ensorceled 2026-03-0413:14 > Gmail added a popup asking the user if they want to unsubscribe when flagging a newsletter with the appropriate unsubscribe headers
Unfortunately close to 100% of the spam I'm flagging causes this popup now :-/
I'm getting a dozen spam a day now on my Gmail account ... I think they're losing the battle.
By sumtechguy 2026-03-0420:55 Pretty sure hotmail/outlook also has the same sort of popup for spam reports. I think accidental would be kind of hard with that popup.
By SoftTalker 2026-03-0419:35 Does gmail still insert ads in the free tier? That would be a reason to keep people reading as many emails as possible.
Agreed. I was an early outlook.com user (was working at MS when it launched, I think internal users got slightly early access allowing me to claim a nicer name than my Gmail) but despite having well over a decade of accounts tied to it got so angry at certain messages never appearing that a couple of years ago I reversed the flow of forwarding and swapped to another account as my primary.
Sounds like it's gotten even worse.
By jonathanlydall 2026-03-0419:56 I always thought of outlook.com as a rebranding of Hotmail (which itself had been continually evolving, was probably actually “Live” at that point), I would expect it is the same (ever evolving) infrastructure.
In which case, people like me with an @hotmail.com address from the 90’s were much earlier users of the outlook.com email boxes than when the domain was “launched” by Microsoft.
