Symptoms
Are SmartActions supported for external email addresses. (I.E. external email with "gmail" trying to action a task by replying "Approve".)
Diagnoses
This is in fact supported in newer versions of K2 Blackpearl.
Resolution
Below is documentation on external email addresses and smartactions: http://help.k2.com/onlinehelp/K2blackpearl/UserGuide/current/webframe.html_K2_SmartActions_Email_Security.html