Short answer
HubSpot's KB: when logging an email, clear the checkbox next to an address under Create as new contacts to log to existing records only. Admins with account access can set the default log and track behavior for all users. Contacts created from logged emails are set as non-marketing if you use marketing contacts. A Super Admin can opt into a beta for contact email logging rules.
1. Account settings
Email Log & Track in my demo portal.

2. Keep personal mail out
See the never log list and log replies only.