[darkarmani]

> This part is the more grey area and interesting to see the ISP inactivity rules.

How is it a gray area? The ISP is telling you to stop sending to the address by using hard bounces. If you continue to send to it, you are spamming.

[IceyEC]

Unless, like their example, you only send email occasionally. In the case that you send 1 email per year, you could hit every one of the listed providers without ever seeing a bounce.

[Gustomaximus]

Exactly this. A typical scenario for those not familiar is often a company has historic lists a year+ old. The company has never sent regular emails and then something interesting happens and you want to inform people that signed up for news some time ago. These inactivity dates are absolutely useful for managing this type of situation.