Email delivery issues with Microsoft Outlook are multifaceted. They stem from aggressive filtering (especially of promotional content), poor IP reputation (resulting from spam complaints, blacklisting, or list bombing), inadequate email authentication (lack of SPF, DKIM, DMARC), sudden volume spikes (particularly from US/CA regions triggering rate limiting), sending to invalid addresses, and complex filtering algorithms that consider IP/domain reputation, content analysis, and user engagement. Deferrals often occur due to temporary server unavailability, but repeated deferrals and blocks indicate more serious reputation or configuration problems. Microsoft's SNDS can help monitor and address deliverability issues.