Changelog
Study: Microsoft’s Azure tenant crackdown and the 550 5.7.237 bounce
Our fourth study, WI-RESEARCH-2026-004. From 29 October 2026 Microsoft began restricting Azure tenants used for cold email, with a new 550 5.7.237 bounce and, on the worst hit, 550 5.7.708. Established tenants at 25 users had no blocks or licence wipes; tenants younger than about two weeks and dense tenants sending 200+ a day were hit.
New
- The study: three fleets through the restriction, our own 12,103-inbox fleet day by day, 1,500+ customer tenants at 25 users, and the dense-tenant segment (its counted figures are still being processed and will be added). Data as CSV and JSON under CC BY 4.0. The study
- The diagnosis: Microsoft appears to restrict cold outbound on tenants younger than 10 to 14 days, while domains on older tenants keep sending on the same sequencer IPs. Prewarmed tenants and aged domains are the answer. Azure inboxes
- The Bounce Analyzer names 550 5.7.237 and 550 5.7.708 and says what to do with each; it also reads three-digit enhanced codes it used to report as "550". Bounce Analyzer
For AI agents
- WarmInboxes Free MCP has a note for both codes and for new tenants being blocked, built on the study’s figures. WarmInboxes Free MCP