Creating this idea for a customer!
This idea is discussed at some length in this community post but it looks like there has yet to be an idea post created for it so here it is!
Currently, HubSpot is set up to create contacts and log emails that are replies to emails from contacts that are already in the CRM. The main reasoning behind this is to protect the CRM from being filled with spam contacts.
That being said, it would be very helpful to allow for a setting or a way to automatically create new contacts from new incoming emails that are not already contacts in HubSpot if it would be possible to do so!