Unable to see another User's emails in 'list email' activities

Set organization-wide default sharing of List Email to 'Public Read Only' 

          Lightning Experience

1. Click the gear icon.
2. Click Setup.
3. Enter Security in the Quick Find box.
4. Click Sharing Settings.
5. Click Edit.
6. Scroll to 'List Email.' The organization-wide default sharing is set to Private.
7. From the picklist, select Public Read Only.
8. Click Save.
 

Classic

1. Go to Setup.
2. Under 'Administer,' select Security Controls.
3. Select Sharing Settings.
4. Click Edit.
5. Scroll to 'List Email.' The organization-wide default sharing is set to Private.
6. From the picklist, select Public Read Only.
7. Click Save.