Do you ever get an email with a winmail.dat attachment and can’t open it?
A winmail.dat file usually appears in your mailbox because different email management programs and email clients process and display the message format differently.
This happens mainly when someone sends you an email through an incorrectly configured Microsoft Outlook email client (or Microsoft exchange server). This is because the sender has a configured Microsoft Outlook Rich Text Format email format and your email client is not configured to process it.
Therefore, it cannot read it and when you start it, the operating system at first prompts you to select an appropriate application to open it.
For example, you can use Winmail Opener to open this file: http://www.eolsoft.com/freeware/winmail_opener.
Another option is to ask the sender to send you the message again in plain text format, which can be read by any commonly used email client. Microsoft has the setup instructions on its website: support.microsoft.com/help/290809.