We recently had an interesting case brought to our attention. Inbound e-mails were being tracked into the CRM, and a corresponding email activity was properly created and linked in the CRM. The Track-in-CRM button was active when the user opened the email. However, the user was concerned that she could not see e-mail messages being tracked in CRM through the default Outlook window (the double-head icon was missing). In other words, no e-mails ‘appeared’ to be tracked in CRM from the main window.
Turns out, the Icon column was missing from the user’s Inbox window.
Hopefully this simple solution will help others who have a similar issue:
1) From the Outlook main window, click on Inbox
2) Right-click on the columns area and select Field Chooser 
3) Drag the Icon field from the Field Chooser to the columns area window 
Before (Missing Icon column):
After:
