High bounce rates, particularly with Verizon (Yahoo, AOL), can be caused by several factors, including issues with email lists (invalid addresses, outdated or purchased lists), deliverability problems, greylisting/rate limiting, poor sender reputation, and changes in email provider policies. Solutions involve list cleaning, double opt-in, email authentication (SPF, DKIM, DMARC), monitoring sender reputation, contacting the ESP for shared IP issues, and validating email addresses before sending.