Outlook.com / Hotmail "550 5.7.1 Unfortunately, messages from [IP] weren't sent ... (S3150)": how to fix

What's happening: Microsoft's consumer mail (Outlook.com, Hotmail, Live, MSN) put the IP that sent your message on its own internal block list. The S3150 code is about the sending IP's reputation at Microsoft, not about your message content. If you send through Google Workspace, Microsoft 365 or a big email service, the IP belongs to them; if you run your own server or a small host, it's yours.

How to fix it

  1. Read the bounce for the IP in square brackets. If it belongs to your email service (SendGrid, Mailgun, a shared host), send them the bounce: they can move you off a bad shared IP.
  2. If the IP is yours, request delisting through Microsoft's sender support form (the bounce links to it). Approval is usually automatic within a day if the IP looks clean.
  3. Before you ask, make sure SPF, DKIM and DMARC all pass for your domain, and that the IP has a matching reverse DNS (PTR) name.
  4. Sign up for Microsoft SNDS and JMRP with the IP to see its complaint data and get notified about future problems.
  5. If delisting is refused, slow down: stop sending to old or bought lists and send only to people who asked for your mail, then ask again after a few days.

Not sure which record is wrong? Check your domain free: it shows which of SPF, DKIM and DMARC is failing, in 5 seconds, no signup.

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