Sudden drops in Microsoft email deliverability stem from several factors, including changes in Microsoft's filtering algorithms, poor sender reputation, high complaint rates, content issues, seasonal volume spikes, and inadequate authentication. Solutions involve monitoring sender reputation through SNDS, implementing SPF, DKIM, and DMARC, maintaining clean email lists, creating engaging content, and addressing spam triggers. Submitting a ticket to Hotmail might be necessary if issues persist. Checking email headers for insights like the X-Microsoft-Antispam setting is also recommended.