A Winmail file is a proprietary email attachment format created by Microsoft Outlook. It often has a .winmail or .att extension and contains the email message and any attachments in a single compressed file. Winmail files are not compatible with other email clients and can appear as unreadable text or an attachment with an unknown format.
Winmail files were introduced in the early days of email to overcome limitations in the way email messages were handled. At the time, email systems were not standardized, and different clients used different methods to format and encode messages. Winmail files provided a way to ensure that messages sent from Outlook would be compatible with other email clients, even if those clients did not support the same formatting options.
