Sender rejection errors and low reputation bounces, particularly with Yahoo, are caused by a multitude of factors, including poor sender reputation, blocklisting, lack of email authentication (SPF, DKIM, DMARC), high complaint rates, spam-like content, invalid email addresses, full inboxes, server issues, high bounce rates, and sending to spam traps. Solutions involve cleaning email lists, implementing email authentication, monitoring and addressing recipient complaints, warming up IP addresses, monitoring deliverability metrics, adhering to Yahoo's guidelines for bulk senders, analyzing rejection messages, contacting ESP support, avoiding spam traps, and whitelisting IPs where appropriate. Yahoo's practice of using 4xx deferrals, which might be mistaken for permanent rejections, adds complexity.