I'd imagine, unless told specifically otherwise, that preview does indeed constitute opening said email...
I'll give my reasons to be cautious unless someone else knows better....
Outlook will happily display emails which are in html format. Now if Outlook or OE defines it's own browser to read said emails then all is probaby well and good, providing you don't manually open it.
However if Outlook uses a plugin of Internet Explorer to preview or read the email then it could be a problem as IE will automatically download executable files of certain file extensions (like .ocx) and run them. Windows tends to use plugins, termed, I think, OLE's for stuff like showing Excel tables in Word etc so I'd be very cautious....