What is the difference between logging and tracking an email?

Tracking an email

A tracked email will send you a notification when the tracked email is opened. You may choose to have all your emails tracked by default by clicking into "settings" on the Hubspot plugin in your email client and checking the box next to email tracking.

Logging an email

When an email is logged it will be added to Hubspot. If a contact with the email address your email was sent to exists in Hubspot, your email will be added to that contact's activity feed. If no contact exists with that email address, Hubspot will create one and log your email to it. 

It is generally not recommended to have all of your emails logged by default. To prevent this, click on "settings" in the Hubspot plugin in your email client and uncheck the box next to "log emails"