Microsoft 365 refuses our mail, IP is on a blocklist
Microsoft runs its own reputation system (SRD) on top of public RBLs. A 5.7.1 or S3140 / S3150 error in the NDR means Microsoft is blocking your IP. Delist via support.microsoft.com/Office/Sender or the 'Office 365 anti-spam IP delist portal'.
Try this first
- 1Capture the exact NDR text. Codes like S3140 (banned sender), S3150 (banned IP), or 5.7.1 (550) tell you the level.
- 2Visit the Office 365 Anti-Spam IP Delist Portal (sender.office.com), enter your IP and a contact email. Microsoft replies within an hour or a day on whether delist is accepted.
- 3In parallel investigate why your IP was flagged: a compromised account sending spam, an open relay, or a PHP script. Don't delist without removing the cause, you'll be relisted.
- 4Enable SNDS (postmaster.live.com/snds) for ongoing monitoring of your IP reputation at Microsoft.
- 5Consider whether to keep sending from your own IP or via an ESP. Below 100k per month an ESP is often more practical.
When to bring us in
If you're blocked again after delisting, there's an outbound issue you can't see (compromise, scripted mail). A mail-flow audit beats another delist round.
See also
- Our emails land in spam for some recipientsAlmost always an SPF, DKIM, or DMARC setting that is wrong or missing, or a sender name that mimics a well-known brand.
- Someone reports receiving phishing emails "from us"Read: spoofing. Someone is abusing your sender name, not necessarily your actual mailbox.
- An email bounces (NDR): delivery failedThe NDR text usually states the exact reason. Reading it is step one.
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