Mail app in OS X is more than just email. You can use Mail app as a RSS reader. Mail app lists RSS feed items similar to email, with a pane displaying the feed content.
If you rely heavily on inbox to manage your emails and todo, you might find it convenient to display incoming RSS feed items in your email inbox. This allows you to manage both emails and feed items with one inbox.
To enable this, right click on the RSS feed for the popup menu. Select “Show in Inbox”. Mail app will now display the feed items in your inbox. To disable, simply select the menu item again to uncheck the “Show in Inbox” option.