I think the movie "You've Got Mail" featured some screen shots of America
Online. I'm not sure if permission was obtained or not, however. If permission
was obtained, it is likely the closing credits would mention this. For example,
something like: "America Online's email browser is the property of
Time-Warner-AOL. Used with permission."
If your movie is a work of fiction, it's safest to design a fictional email
browser and give it a name that does not exist in real life.
On the other hand, if your movie is a documentary about Outlook or Microsoft,
use of Outlook may then constitute "Fair Use."