Lots of people and ESPs have had this issue over the last few days. Apparently,
Microsoft is aware of the problem and is working on a fix.
We have started seeing improvements in deliverability today.
[Regards,]
Areeb Majeed
Co-founder,
Maileroo
[icon] maileroo.com [icon] [email protected]
[facebook](https://facebook.com/maileroo)
[linkedin](https://www.linkedin.com/company/maileroo/)
[twitter](https://twitter.com/maileroodotcom)
[__tpx__]
Maileroo Group Pty Ltd| Level 10, 440 Collins Street, Melbourne VIC 3000
> On 15/05/2026 8:01:03 PM, [email protected]
> <[email protected]> wrote:
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> Today's Topics:
>
> 1. Persistent Outlook.com S775 rate limiting since May 8?
> (Fernando MM)
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Thu, 14 May 2026 22:57:02 -0300
> From: Fernando MM
> To: Mailop
> Subject: [mailop] Persistent Outlook.com S775 rate limiting since May
> 8?
> Message-ID:
>
> Content-Type: text/plain; charset="utf-8"
>
> Hi all,
>
> We’re trying to determine whether others are experiencing similar
> Outlook.com delivery issues that started around May 8.
>
> Multiple IPs suddenly began receiving the following error: “451 4.7.650 The
> mail server [IP] has been temporarily rate limited due to IP reputation
> (S775).”
>
> The issue appears to be persistent. Retrying after 12, 24, or even 48 hours
> has no effect, and no messages can be delivered from the affected IPs.
>
> We also opened tickets through https://olcsupport.office.com/. Support
> consistently replies with messages similar to:
>
> * “We have implemented mitigation for your IP (XXX), and this process may
> take 24–48 hours to replicate completely throughout our system.”
> * “Please try sending emails again within the next 12 hours and let us know
> if you continue to experience issues.”
>
> However, after waiting the requested time, the exact same error continues.
> When we follow up, we receive essentially the same response again, which
> leaves us stuck in a repeating support loop.
>
> Is anyone else seeing similar persistent S775 reputation throttling issues
> since May 8, particularly in cases where mitigation was reportedly applied
> but had no visible effect?
>
> Thanks,
>
> Fernando M M
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