You have to install php5-imap and postfix. These are not installed by default on Ubuntu desktop or Ubuntu server.
sudo apt-get install php5-imap
sudo apt-get install postfix
The postfix install will ask a bunch of questions. The defaults will suffice for most of them but remember to give it the domain of your OSTicket email address (mine is helpdesk@my-mis.com so the domain is my-mis.com)