When adding the condition, Active is true, to the ACL for a task record, the icon for the Email Client is not displayed--not even to ITIL users.
This problem has been fixed in Calgary Patch 5, Dublin, Eureka.
For more information please consult the master problem ticket:
This can be worked around by removing the condition, active=true, and setting alternate conditions on the task State.
Condition: State IS NOT Closed.