As per this article, email recipients must be users in the portal: Create a sandbox and deploy changes to production
“You can send one-to-one test emails and marketing emails from a standard sandbox account to existing sandbox users. If you send emails to contacts that are not sandbox users, the emails will not send because they are not users in the standard sandbox account. You can add HubSpot users within your sandbox account. Emails sent from a standard sandbox account will come from a different transactional IP address than the main account uses.”
To send a contact a transactional email from a sandbox or developer account, the email associated with the contact will also need to be associated with a user in the same developer account or sandbox as @karstenkoehler mentioned in his reply.
I’d recommend referring to his reply to see if this helps resolve your issue If you’re still running into this issue after trying out the suggested steps, can you please let us know?