I previously used a service that sent me Slashdot articles as individual e-mails. All I had to do was give them the RSS url, and they handled the rest. On the Gmail side, I simply made a filter, labeling these e-mails as "News".
Once I stopped using the service, I realized that I had more than 20,000 such e-mails stockpiled. While I could delete all "News" e-mails, there are some that I've forwarded to friends, sometimes receiving multiple replies. These conversations are valuable - I would like to keep them.
As far as I can tell, there's no way to search for conversations that have more than one e-mail in the thread. Up until now, I've been deleting them page by page, carefully de-selecting the conversations that have more than one e-mail; however, it will take far too much time to complete. Is there an easier way that I'm missing?