Problem with emailing the requester when a request is closed

Our company is evaluating your help desk software and has run into a snag.

When a new ticket is opened the requester receives an email; hence the "Acknowledge Requester by email when a request is received" option is marked.

However when one our technicians closes the ticket a confirmation email is NOT sent to the requester notifying them the ticket has been resolved and close.

We verified our mail server settings are correct and the option to "Email Requester when a request is closed" is marked. What could be the problem?

The demo is ServiceDesk Plus Version : 4.0.0 Build Number : 4001

Thank you in advance,

M~