To improve email deliverability to Hotmail/Outlook, even with proper authentication and opt-in, a multi-faceted approach is required. Maintaining a positive sender reputation is crucial, which includes monitoring IP reputation through SNDS and JMRP, keeping complaint rates low, resolving blacklist listings, and gradually warming up new IPs. Technical aspects include ensuring valid SPF, DKIM, and DMARC records and, potentially, Sender ID authentication. Content quality is also essential; avoid spam trigger words, use clean HTML, and maintain a good text-to-HTML ratio. Focus on list hygiene by validating email lists regularly and removing inactive addresses. Active monitoring and response to feedback loops is important. Subscriber engagement matters; encourage recipients to add you to their address book and reply to your emails. List bombing prevention measures are vital for avoiding deliverability issues. Checking that links in your email point to domains with a good reputation. Contacting SNDS/JMRP is crucial if deliverability issues still persist.