Log an Email You Sent or Received Outside the Application
Provided you start an email conversation from an account, a contact, or a lead, both outgoing and incoming emails are logged automatically in Activities as a thread. The most recent email appears at the top. If you send the initial email from outside the application, from a personal email account, for example, then the email thread doesn't get automatically logged in Activities. You can still track any outgoing and incoming email in Activities by manually entering it.
Results:
Each email you log manually appears as a separate entry in Activities. You don't get the threads you have with automatic email logging.