Once in a while you may receive an email with an attachment called “winmail.dat”. How do you open it?

Short answer: The LookOut addon for Thunderbird:

https://addons.mozilla.org/en-US/thunderbird/addon/4433

Long answer:

Microsoft wanted to offer “rich” text features in their emails. The approach they took was to send a plain text version of the message and a version coded into a form of Rich Text Format. If the mailer at the other end could handle the Rich Text Format version they would see that, otherwise they would see the plain text. To do this, they used their own method, called the MicroSoft Transport Neutral Encapsulation Format, or MS-TNEF. Essentially a file called WINMAIL.DAT, which is just a standard MIME encoding of a Rich Text Format version of the message, is included with outgoing mail. http://blog.myfenris.net/?p=181.

So to answer some questions:

Q. Why is this happening?
A. Microsoft likes to break standards in its pursuit of a monopoly. This has been a very profitable strategy.

Q. Can I stop the person sending the email doing it again?
A. They need to avoid Rich Text for their emails or their Microsoft email program will automatically violate email strandards. From Wikipedia: “Within the Outlook email client TNEF encoding cannot be explicitly enabled or disabled. Selecting RTF as the format for sending an e-mail implicitly enables TNEF encoding, using it in preference to the more common and widely compatible MIME standard.” (http://en.wikipedia.org/wiki/TNEF).

Q. How do I open the attachment?
A. If you are using Thunderbird the easy answer is the LookOut extension. Otherwise, see below (with big thanks to: http://blog.myfenris.net/?p=181):

Install tnef:
sudo apt-get install tnef ytnef libytnef0 libytnef0-dev
Save proprietary, standards-non-compliant attatchment to your desktop
tnef --file=Desktop/winmail.dat
Open home folder to see extracted attachment – which could be a jpg or anything that is an attachment really.