Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Adding RSS Feeds to Mail
Mac OS’s Mail enables you to read not only emails but also RSS feeds. For example, to subscribe to the CNN US news, do the following:
1. Go to the http://edition.cnn.com/US/ page.
2. Click the RSS icon in the Address Bar to display all RSS feeds available for this page.
3. Click the + button to create a bookmark and select Mail in the window that appears.
4. Click Add.
5. Mac Mail will create a new folder that will receive all of your RSS feeds.
6. Read the RSS feeds as ordinary mail, and click Read more to navigate to the webpage and visit the news source.