[andrewaylett]

> residential IPs cannot into email cause spam

Strictly, that's dynamic IPs. The Venn diagram may almost be circular, but not quite. Part of the point being that it's much easier to block static IPs, while a spammer on a dynamic IP will evade any blocks when their IP address changes.

My in-laws outbound email broke recently because, being on a dynamic IP, they'd been set up to send through their ISP's smarthost. Then they changed ISP :P.