Go to Admin > Workflows > Automations > New rule under the Ticket updates tab.


Sample rule:


When an action performed by: 

Agent or Requester


Involves any of the following events:

Status is changed from Any Status to Resolved 

Status is changed from Any Status to Closed


On tickets with these properties: 

if Source > is any of > Portal


Actions: 

Send email to requester (configure your email)