If you use an e-mail client other than Outlook and receive winmail.dat files from senders using Microsoft Outlook (1997, 2000, 2003, 2007, 2010), then you won't be able to open them without WinMail Viewer. Winmail.dat files contain attachments and messages in rich text format and with WinMail Viewer, you can open them. Winmail Viewer allows you to view attachments of a winmail.dat, open single attachment files in external applications and extract and save files contained on the Winmail.dat attachments.