News and Mail transfer require that a binary attachment is "encoded" before it is sent. And they are "decoded" after they have been received. Normally all this is done by your newsreader (or mail-program). You don’t see it. Most don’t even know it. yEnc files are formatted in a coding system that is up to 40% smaller than UUencode and Base64, and also include error checking and multipart file support. This means that posts are smaller, and take less time to download.
yEnc Format Files - What are the Problems with yEnc? (Page 3 of 5 )
If yEnc results in smaller files for downloading and less overhead, then what is the problem? Usenet users who for years used news readers like Outlook Express that do not support yEnc are the most vocal critics. This means that they have to download and learn how to use a different news reader.
There is an ongoing debate on why yEnc is bad for Usenet, much of that centering on how the messages have to be formatted. This would be a problem that is solved by the encoding program, such as yEnc32. You can read about it on sites like Slashdot and in groups such as alt.binaries.news-server-comparison.
What Do I Need to Decode Usenet Files
So you haven’t been sucked in by the anti-yEnc movement, and want to try decode the content in this format that has become the standard. To do this, you need a news reader that accepts this format.
As far as I know, Outlook Express, the news reader that comes with most versions of Windows, does not decode yEnc files. Click here (.doc) for a list of some tools that support yEnc. The list includes tools for Windows, Linux/Unix, and Macs.