Fixing the Outlook.com block: "messages weren’t sent" (S3140 / S3150)

The S3140/S3150 codes are Outlook.com (the consumer side of Microsoft mail) telling you the sending IP has poor reputation. It is a soft-worded but firm reputation block, not a record error.

What it means

Outlook scores every sending IP. When the score drops — from complaints, a volume spike, or a listing on a blocklist Outlook consults — new mail is refused with S31xx and a link to their troubleshooting page.

Why it happens

Recipients marking your mail as junk is the biggest factor, followed by sending to invalid or trap addresses and sudden increases from a cold IP. A public blocklist hit (often Spamhaus) frequently accompanies it.

Diagnose it

Run the blocklist check on the exact IP in the message. If Spamhaus or Barracuda also list you, clear those first — they often drive the Outlook score. Confirm rDNS and authentication while you are there.

Fix it

1. Reduce complaints: honour unsubscribes instantly and stop mailing unengaged recipients.

2. Clear any public blocklist entries and set correct rDNS + SPF/DKIM/DMARC on the IP.

3. Enroll in SNDS + JMRP, then use the Outlook.com sender support / mitigation form for the IP.

4. Warm the IP back up with lower, steady volume to rebuild reputation.