Your employer's system administrator probably disabled the preview pane on every employee's computer because it is a security threat.
Phishers have created ways to find out personal, company, and internet access information from your computer if their phake email is opened in the preview pane as the "next message." Most IT security departments disable the preview pane on all company computers.
It is also against most security policies to leave email software open when you are not present at the computer. You are inviting disaster for your company with your unsecure computer use.