How MailSync creates or updates HubSpot contacts
When a contact already exists in HubSpot with the same email address MailSync will simply add the SendGrid email events to the timeline on the existing contact.
If a contact does not exist in HubSpot a new contact record will be automatically created so that the events can be logged to the timeline. The contact is created with only the email address property in HubSpot.
We do not sync any other contact properties from Sendgrid as it does not come through the webhook feed.
If you would like more control over what get's sync please take a look at our Sub User config guide here.