How to handle bounced emails

### Executive summary This document explains ISP hard blocks in email delivery—permanent SMTP 554 rejections applied when messages trigger policy or reputation checks. It outlines the typical progression from spam-folder placement to temporary failures and eventual hard blocks, common causes (URL/body content blocks, authentication failures, poor IP/domain reputation, blocklist listings), example bounce text, and remediation guidance including resolving root causes and contacting the blocking network (noting some providers like Gmail may offer no remediation path).