Using Bcc for Salesforce Activity Tracking:
Users can leverage SFDC Bcc functionality to bring over activities from an outside email account. It is important to note that from whichever account you pull the unique bcc from, the activities will be tracked against.
For example, if khakie@swantide.com is a user in Salesforce, and bcc's her unique SFDC code on another email (test@swantide.com) any activities that pull through from test@swantide.com will be associated with the khakie@swantide.com user in Salesforce
How to find your unique Bcc:
Navigate to the profile icon in the upper-right hand screen of your Salesforce and click "Settings"
Search in the quick find box "My Email to Salesforce"
Grab the Email to Salesforce Address bcc email and use this on an individual or automated basis to pull in activities and associate them to the user account you are logged into