split screen now when email is opened

Just did the new update on out iMac & now our mail shows a split screen; like the old "preview pane" in Outlook. I don't like the mail being opened when I open the program because when you use to use a "preview pane" it allowed for the email to be opened and therefore if a virus was there you just let it out. I'd like to go back and just see my mailboxes & who/what & subject of the emails I received. Turning the program back to the "old way."