HubSpot Sales is able to track email opens by embedding a one pixel invisible image into the message once it is sent. Links are tracked by appending tracking to the end of any hyperlink that is included in your email.
When your recipient views the email and the images load, you receive a notification that the email has been opened. You can see this information in your HubSpot activity stream as well.
Please note: an open notification will fire anytime the tracking image loads, even if the email was simply in a preview pane in Outlook but was never actually opened.
HubSpot Sales tracking can sometimes be deactivated if the recipient's email client doesn't allow the tracking pixel to load. For instance, if the recipient formats their email in plain text, if their email client doesn't automatically download images, or if their corporate filter strips away all images for incoming emails, HubSpot tracking will be deactivated. This shouldn't happen often, but can happen from time to time.
As for spam filters, the only time HubSpot Sales would cause your mail to go to the spam folder is if the recipient has a very strict spam filter where they do not allow images or link tracking to be present in emails. However, both of these tools are quite common in the industry, and the majority of email clients will not view your email any differently than before.