Symptoms
No response from SmartAction Listener when action the task via email

Diagnoses
There will be a Warning thrown from the console window indicating that the SmartAction Listener processing has been halted due to Spam Restriction

Resolution
Turned out this is due to email spam filter. Resolved after it was turned off:-
Look into "Security Recommendations" section in https://help.k2.com/onlinehelp/k2blackpearl/userguide/4.6.6/webframe.html#k2_smartactions_email_security.html
Followed by Blackpearl service restart.