A file that contains RTF format codes in order to support rich text in e-mail messages. WINMAIL.DAT is a UUencoded file that is sent from a Microsoft Outlook or Exchange e-mail client. If the recipient e-mail system is not Exchange or cannot decipher RTF, WINMAIL.DAT winds up as an attachment that is not processed. Outlook may also be configured to use WINMAIL.DAT as a wrapper to encapsulate attached files. See rich text and UUencode.