Yes. By default, contacts that were originally added to the HubSpot CRM can be emailed from the marketing platform.
Acceptable Use Policy
However, you must obtain permission from these contacts to send them marketing email; if contacts originally added to your HubSpot CRM have not opted in to receiving your marketing email, sending them email without their consent violates HubSpot's Acceptable Use Policy.
If you have double opt-in set up, these contacts will need to opt in to email before receiving marketing emails. Contacts will need to fill out a form on your site and confirm in the follow-up email that they'd like to receive marketing emails from you.