What are the risks of ignoring hygiene?

Ignoring hygiene leads to permerrors such as permanent hard bounces and transient errors such as temporary soft bounces. It also results in IP reputation blocks, domain reputation blocks, and listings on blocklists such as Spamhaus. Poor hygiene reduces ROI. When you send to people who no longer want your messages your conversions collapse while your cost per send stays the same. It is like filling your cargo hold with rotted goods. You still pay for the space but none of it delivers value.