Microsoft Resolves Classic Outlook Email Delivery Bug Affecting Outlook.com Users
What Happened — A server‑side defect in Classic Outlook caused non‑delivery reports (NDRs) for users sending mail through Outlook.com when the profile was linked to another Exchange account or had a duplicate SMTP contact. Microsoft deployed a fix on April 3 2026.
Why It Matters for TPRM —
- Email continuity is a core service for most third‑party vendors; disruptions can cascade to downstream business processes.
- Unresolved NDRs may mask phishing or spoofing attempts if users assume delivery failures are normal.
- The issue highlights the need to monitor vendor‑provided service health and maintain fallback communication channels.
Who Is Affected — SaaS email providers (Outlook.com), enterprises and individuals using Classic Outlook with linked Exchange accounts.
Recommended Actions —
- Verify that all Outlook clients are updated to the latest version or switch to the New Outlook/web client.
- Monitor NDRs and error codes (0x80070005‑0x0004dc‑0x000524) for any lingering delivery failures.
- Document the incident in your vendor risk register and confirm Microsoft’s remediation status.
Technical Notes — The bug manifested as NDRs when Classic Outlook attempted to send mail via an Outlook.com account linked to another Exchange mailbox or when a duplicate SMTP address existed in an Exchange Online contact. The fix was a server‑side configuration change; no client‑side patches were required. Source: BleepingComputer