Recently for my son's 8th grade fund raiser, someone sent out a document for a flier to have everyone print out and distribute. But most people could not read it. The person sent it again, and it was the same unreadable mangled file.
It was a winmail.dat file. I couldn't find any software that could open it.
After searching for it, I found out that when Outlook is sending a rich text or HTML format email it encodes it as a winmail.dat file and includes the file attachment as well inside of it. Only Outlook can decrypt this file and remove the file inside. If Outlook sends the email as plain text, every email client can read it and find the attachment.
As it turns out there are a lot of winmail.dat extractors out there. The one I found was Winmail Opener that was free and works with Windows, and since I use Ubuntu it worked with WINE. I extracted the DOC file and then emailed it to every parent on the list so they had their flier to print out and distribute.
Now some email clients have been updated to open this file, but not all of them. If you are told the problem or failure is on your end, don't believe it but download one of these winmail.dat opening programs listed above and then extract the file from it. Microsoft seems to only care about selling software and not following email standards and helping people. Most people using Outlook as an email client don't know the harm it causes others who cannot read the emails they send.
This is of course not the first time Microsoft has done this sort of thing and it won't be the last either. But because Microsoft has a monopoly by preinstalling Windows on every PC sold, they can do whatever they want.This allows them to gain IT corporate sales and push their Office and Back Office software on businesses and then when it doesn't work with other software out there because it does not follow standards, it hurts everyone else.