Possibly the S3150 block is an escalation of the below "temporarily rate limited due to IP reputation" error? I've tried to correlate the two but have not yet found a distinct connection, i.e. that it might precede the S3150 block. I also don't see the error documented on the ' https://aka.ms/postmaster' web site that is included in the rejection, unfortunately.
451 4.7.650 The mail server [ip_address] has been temporarily rate limited due to IP reputation. For e-mail delivery information, see https://aka.ms/postmaster (S775) [trimmed] David Landers Deliverability Operations Engineer [image: %(logoAlt)s] <https://www.twilio.com/?utm_source=email_signature> EMAIL [email protected] On Mon, Jan 26, 2026 at 3:14 AM Benoit Panizzon via mailop < [email protected]> wrote: > Hi > > Same issue here, we once again hit the dreaded: S3150 error last Friday. > > Filled out the form - came back: we do not block any of your IP's > listed. > > Replied once again with actual mail-logs showing the blocking. > > This morning got the surprising reply: We found and mitigated the issue. > > What we still would really like to find out is: What is the root cause > of the S3150 issue? > > Is it Microsoft customers reporting spam mails they forward from their > accounts on our platform to their Microsoft email addresses? > > Is it the few spam mails that are sent via phished accounts on our > platform before our countermeasures manage to block the accounts? > > Is there anything we can do to prevent S3150 from happening repeatedly? > > > In my very limited sample size of 2, Microsoft's first reply is > > automated and claims everything is peachy keen. However, if you reply > > again your issue is actually reviewed, either by a bot or a human. > > Mit freundlichen Grüssen > > -Benoît Panizzon- > -- > I m p r o W a r e A G - Leiter Commerce Kunden > ______________________________________________________ > > Zurlindenstrasse 29 Tel +41 61 826 93 00 > CH-4133 Pratteln Fax +41 61 826 93 01 > Schweiz Web > https://urldefense.com/v3/__http://www.imp.ch__;!!NCc8flgU!aBctW2h9Gd366sl6SfRecmevr92tv6_CEPDg2HOF5tCH5R3wtcR3AHLysMb0JGCL4blY44g1gtfA2w$ > ______________________________________________________ > _______________________________________________ > mailop mailing list > [email protected] > > https://urldefense.com/v3/__https://list.mailop.org/listinfo/mailop__;!!NCc8flgU!aBctW2h9Gd366sl6SfRecmevr92tv6_CEPDg2HOF5tCH5R3wtcR3AHLysMb0JGCL4blY44h0kvcnqQ$ >
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