Updated June 10th, 2019.
Sometimes you get an email with an attachment called “winmail.dat.” You’re never able to open it– unless you use Josh Jacobs’ excellent “TNEF’s Enough” program. It’s available on the Mac App Store for free via this link.
Easiest way to use it: put TNEF’s Enough into your Dock, then drag winmail.dat files from your email program to the TNEF’s Enough icon. The rest you’ll figure out.
TNEF’s Enough allows Macs to read and extract files from Microsoft TNEF stream files. The files are usually received by SMTP based e-mail programs from Microsoft Exchange and Microsoft Outlook users. The SMTP based e-mail program will usually receive either a MIME attachment named “winmail.dat” or a MIME attachment with the type “application/ms-tnef.”
The file is a rich text (or MAPI) message that is sent from Outlook to Exchange. When Exchange sends the message to an outside server it writes the MAPI message as a MIME attachment. The unfortunate side effect of this plan is if the Outlook user has someone in their address book as a person who can receive “Rich Text” then the user will receive the TNEF file whether the user uses Outlook or not.
The app is free, but you can donate to the developer if you’d like to, and considering how much trouble TNEF’s Enough saves us, I think we all should do it. See this web page for more information.