On 25 July 2012, Apple released Mountain Lion (OS X 10.8) as the next major version of their operating system.
Some users complained about losing all of their saved RSS feed data from within the Mail client after installing Mountain Lion.
Although it’s no longer visible in Mail, old RSS feed content previously downloaded can be found under User > Library > Mail > V2 > RSS.
The Library folder is accessible under the Go menu in Finder when you hold down the Option key.
For RSS alternatives, read Five Ways To Replace Safari’s RSS Reader In Mountain Lion.