Note:
Server Address block below hidden with 198.51.100.0 . All other address blocks are as provided by the log.
The email feedback stats:
> LOG: MAIN cwd=/usr/local/cpanel/whostmgr/docroot 4 args:
> /usr/sbin/exim -v -M 1vTFQx-000000006bI-3Seh delivering
> 1vTFQx-000000006bI-3Seh LOG: MAIN Unfrozen by forced delivery LOG:
> MAIN Sender identification U=mailnull D=-system- S=mailnull
> Connecting to outlook-com.olc.protection.outlook.com [52.101.41.25]:25
> ... non-TFO mode connection attempt to 52.101.41.25, 0 data connected
> SMTP<< 220 SJ1PEPF0000231A.mail.protection.outlook.com Microsoft ESMTP
> MAIL Service ready at Wed, 10 Dec 2025 16:42:29 +0000
> [08DE2E6E2E888588] SMTP>> EHLO basic.coreone.co.uk SMTP<<
> 250-SJ1PEPF0000231A.mail.protection.outlook.com Hello [198.51.100.0]
> 250-SIZE 49283072
> 250-PIPELINING
> 250-DSN
> 250-ENHANCEDSTATUSCODES
> 250-STARTTLS
> 250-8BITMIME
> 250-BINARYMIME
> 250-CHUNKING
> 250 SMTPUTF8 SMTP>> STARTTLS SMTP<< 220 2.0.0 SMTP server ready LOG: MAIN [52.101.41.25]
> outlook-com.olc.protection.outlook.com Received TLS cert (DN:
> '/C=US/ST=Washington/L=Redmond/O=Microsoft
> Corporation/CN=mail.protection.outlook.com') status response, itself
> unverifiable: certificate verify error SMTP>> EHLO
> basic.coreone.co.uk SMTP<<
> 250-SJ1PEPF0000231A.mail.protection.outlook.com Hello [198.51.100.0]
> 250-SIZE 49283072
> 250-PIPELINING
> 250-DSN
> 250-ENHANCEDSTATUSCODES
> 250-8BITMIME
> 250-BINARYMIME
> 250-CHUNKING
> 250 SMTPUTF8 SMTP|> MAIL FROM:<> SIZE=14552 SMTP|> RCPT TO:<[email protected]> SMTP>> DATA SMTP<< 550 5.7.1
> Unfortunately, messages from [111.111.111.111] weren't sent. Please
> contact your Internet service provider since part of their network is
> on our block list (S3150). You can also refer your provider to
> http://mail.live.com/mail/troubleshooting.aspx#errors. [Name=Protocol
> Filter Agent][AGT=PFA][MxId=11BC5CDC449C7EB0]
> [SJ1PEPF0000231A.namprd03.prod.outlook.com 2025-12-10T16:42:31.233Z
> 08DE2E6E2E888588] SMTP(closed)<< SMTP+> QUIT SMTP(TLS
> shutdown)>> SMTP(shutdown)>> SMTP(closed)<< SMTP(close)>>
> cmdlog:
> '220:EHLO:250-:STARTTLS:220:EHLO:250-:MAIL|:RCPT|:DATA:550:QUIT+' LOG:
> MAIN ** [email protected] R=dkim_lookuphost
> T=dkim_remote_smtp H=outlook-com.olc.protection.outlook.com
> [52.101.41.25] X=TLS1.2:ECDHE-RSA-AES256-GCM-SHA384:256 CV=yes: SMTP
> error from remote mail server after pipelined MAIL FROM:<> SIZE=14552:
> 550 5.7.1 Unfortunately, messages from [198.51.100.0] weren't sent.
> Please contact your Internet service provider since part of their
> network is on our block list (S3150). You can also refer your provider
> to http://mail.live.com/mail/troubleshooting.aspx#errors.
> [Name=Protocol Filter Agent][AGT=PFA][MxId=11BC5CDC449C7EB0]
> [SJ1PEPF0000231A.namprd03.prod.outlook.com 2025-12-10T16:42:31.233Z
> 08DE2E6E2E888588] LOG: MAIN Frozen (delivery error message)
Microsoft rejected our email due to the IP address is on a block list.
We are not on any blacklist (I am aware these are different), and we pay to be Whitelisted.
Further, using https://sender.office.com claims we're not on a MS Blocklist despite responses showing we are.
Nothing has changed on the server settings as far as we're aware.
Is there anything we can do or check to help Microsoft 365 accept delivery of our emails?
IP addresses here are obscured.
EDIT: We have reported to their Support infrastructure but had no reply/response.
EDIT 2: Out MXToolbox Site assessment reads perfect; no warnings no errors.