I'm reading the ZeroBounce docs and it seems very relevant. Look at this step: "We recheck all unknown emails using IPs from different geographical locations".

I'm reading the ZeroBounce docs and it seems very relevant. Look at this step: "We recheck all unknown emails using IPs from different geographical locations". This matches exactly what this article describes as getting these emails from a range of locations. The step before that is just "Proprietary Technology", which sounds like a good cover for what's going on here. How else are you testing an email address after between "real time SMTP server check"?