So the outlook add-on is having you log into outlook. HubSpot is not using or storing your login. You are just connecting the two apps just like you do when you log into outlook on your phone and allow your phone to send and receive emails. You have control over this sending and receiving from HubSpot.
HubSpot is used by over 100,000 companies worldwide. Their security is SOC 2 certified. That is the same level that healthcare is required to have.
You can read more about security here: “Security, Privacy, and Control”
HubSpot does not sell your data. Only you have access to your metadata. HubSpot only showcases the larger numbers that it can see; i.e. email sends, number of users, etc. None of your personal data is sold to third parties.