This address does not automatically populate you need to manually enter your drop box email address into the Bcc: if you send an email out via your email client to enable a copy of that email to be stored inside your CRM account. 

Your drop box address will be:

If you are emailing a contact whose email address is already stored against their contact record inside your CRM database then by using your email drop box address in the Bcc: field this will put a copy of the email you send into the contacts CRM record - where it will be shown inside the History section.