Outlook Extension / Filing previously sent emails into CiviCRM
You can also use the email processor, described at https://docs.civicrm.org/sysadmin/en/latest/setup/civimail/inbound/#autofiling-email-activities-via-emailprocessor.

You can either set it up where you make a subfolder in your outlook and then anything you drag in there gets filed into CiviCRM, or you can set up a separate shared email address and forward emails to it to have them go into CiviCRM.

The downside to the first one is you have to give civi access to your own mailbox by storing your password in civi.
The downside to the second one for emails you've already sent is that they all get filed under your own contact, but if you're sending a new email you can bcc the shared address and then it gets filed on the correct contacts.