HubSpot sends emails on your behalf, and if the email bounces (can not be delivered) HubSpot receives the bounceback message. Email servers explain why the email bounced within the bounce message. The server response is often overlooked by the untrained eye. However, HubSpot has a framework for categorizing these bounce messages.
So what's the point, why bother? Welp, Every time you hard bounce an email it hurts your sending reputation a little bit. A great deal of hard bounces can jeopardize the deliverability of contacts that are not invalid, and want your email. So, HubSpot built this framework to avoid bouncing emails to the same contact multiple times, to shield your sending reputation from unnecessary hard bounces.