The junk folder is Microsoft Outlook's and Windows Mail's term for the spam/bulk folder. When a receiving mail server or client-side filter routes an email to the junk folder, the recipient almost never sees it — the message is functionally equivalent to a missed delivery.
Why junk folder placement damages warm-up:
ISPs track how often a sender's messages are routed to junk. High junk rates signal to Microsoft's filtering systems that the sender is low-trust. Once the junk rate rises, it becomes a self-reinforcing cycle — more mail gets filtered, fewer real recipients see it, engagement drops, and filtering tightens further.
Junk vs. Inbox — what drives routing on Microsoft:
- IP and domain reputation: Tracked via Microsoft SNDS
- Authentication: SPF, DKIM, and DMARC must all pass
- Complaint rate: Recipients marking mail as junk via Outlook's "Report Junk" button feeds directly into Microsoft's filtering signals
- Engagement history: Prior opens, replies, and folder moves from Outlook users influence routing
- JMRP enrollment: Joining Microsoft's Junk Mail Reporting Program provides complaint data back to senders — see JMRP
During warm-up: Monitor Microsoft SNDS to see the junk/inbox split for your sending IP across Outlook.com and Hotmail recipients. A healthy warm-up should show the junk rate declining toward green as reputation builds.