Emails can be created outside of HubSpot and tracked within HubSpot using the SMTP API.
Please note that in order to use this API, you must purchase Transactional Email as an add-on, which includes use of a unique, dedicated IP address. See more information on developers here: SMTP API Overview
For emails sent to contacts outside of HubSpot, contacts will show up as unlinked email addresses on the screen. SMTP API does not currently enable sending emails built with the HubSpot email editor.
What does the SMTP API mean? How does it relate to my other database as it pulls in information? The SMTP API is a way of sending automated transactional email that is triggered by specific criteria, such as making a purchase on an e-commerce website. The SMTP API is used to integrate with any internal or third-party systems to both trigger the email and incorporate data stored outside of HubSpot (e.g. shipping data, purchase price, etc). The email is sent from your system, but is wrapped with the HubSpot tracking codes that allow full engagement tracking and measurement, so you can see the performance within HubSpot.