The challenge is that Hubspot defines "Unengaged" as those who have not opened the last 11 emails. I'm trying to determine the list of people who have never opened a single email.
Creating the filters mentioned reduces my list by 94%, which is not accurate.
>>"The manual way is: "recent Sales Email Open Date" = "Unknown"?, then to add a property to those emails like 'graymail = yes'."
Ok, no.
No need to 'manually' add anything to anything. The filters shown will display all contacts to whom you've sent email (tracked by HubSpot), but for whom NO RECENT SALES EMAIL OPEN DATE has been triggered (in HubSpot). Attempting to do this manually is a complete waste of time.
That being said, email open rates in general are an unreliable indicator of engagement. This technological limitation is not unique to HubSpot.