There has been a Code change in 7.3 and the following needs to be done to get resolution/closure notifications working.
Within designer you need to go to File – Tables - Edit and then choose the table “CALL CLOSE”.
Modify the table and you should be able to click on the QUEUE TABLE button.
This will bring up a window called Queue table identifier . You need to ensure that AUTOEMAIL is selected as the application and that the insert and update boxes are checked. Now apply the setting, log out of Designer and log back in to Helpdesk and resolution and closure should send out emails.