Comcast email throttling is primarily caused by sending emails too quickly, especially from new IPs, and by poor sender reputation, often resulting from spam complaints. Improving open rates involves enhancing sender reputation through list cleaning (removing unengaged subscribers, using double opt-in), properly authenticating email (SPF, DKIM, DMARC), and monitoring blocklists. Additionally, crafting compelling subject lines and preheader text, segmenting email lists, personalizing emails, testing send times, and maintaining a consistent sending schedule are crucial for increasing open rates.