This excerpted from a discussion on this topic:
This is an option that could be added to the Sales settings page where you can retrieve the logging email address used by HubSpot for your particular hub. Why not just have a checkbox that says “create new contacts when they do not exist”? It would be a simple user interface addition AND the logic code in the logging process should be as simple as “if the checkbox is unchecked, then skip the code that adds the contact”. Similarly, the code could be added on an individual email basis where you choose to log it or not.
Is there a specific reason why we do not want to just uncheck the Log in CRM box on an email to prevent these contacts from being created? Yes, many:
a) we do not always remember to uncheck the logging / tracking boxes with every email and by default the behavior to turn it on and turn it off with each email sent is very cumbersome.
b) a lot of emails we send go to a “contact” we have in hubspot for a company which we DO want to log. However, we may need to CC vendors or support personal in that same email. Those additional CCs do not need to be put in there at all and the option would allow them to be skipped.
There are many other situations I can think of where this is the case. My last post in the community thread details further.
Please add this to your list of planned enhancements to the Hubspot platform