These are the mailing lists that are private (protected by your credentials) and hence do not expose a direct feed url. You can only get them into your inbox. These are also the lists to which you seldom reply to, and its mostly skimming through articles to keep you informed.
I am subscribed to one such list and I quickly created a mail account to which I directed mails from the mailing list. I tried a Gmail account but the IMAP on gmail somehow seems to be unreachable. I created an AOL mail instead and linked that account to my account on mailonfeed.com.
The only problem with adding this "private" feed to reader is that is uses HTTP authentication and most readers cannot fetch content when HTML authentication is enabled. A better way would have been to add a secret long random string to the end of the feed URL to make it private (like Flickr uses for email uploads)
Till then, just happy with saving some spam from my inbox. :)