Hi @randaldehart,
At HubSpot we believe that if a contact has not specifically opted into email communication, then you should not email them at all. Accordingly, if there are addresses on your list that you know were purchased or have not explicitly opted in then I recommend deleting them.
That being said, HubSpot automatically checks to see if email addresses you import could be invalid. I say this because we examine the syntax of the email and if any of them are clearly invalid we will prevent you from adding/emailing them. Meanwhile, a list hygiene software will actually ping the servers of your addresses to see if they are valid. Additionally, if an email has bounced previously from another HubSpot account, then we will mark it as such to prevent you from sending to them, which would harm your sending reputation.
That being said, if you use legitimate means to acquire your contact lists then you should not need to use list hygiene software.
I recommend running an email reengagement campaign, which similar to your suggestion, helps you identify dormant contacts, see if they want to continue receiving emails, and remove those who remain inactive. Here is a guide on how to do this: HubSpot Login and Sign in
Rami