Please go to your feed’s setting page (go to My Offered Feeds and click on the Settings-icon next to the feed) and select the tab “Sending options”. There you can change the subject line of the emails.

You can set a static subject line, or make it dynamic. For example, if you include “%laststory%” (without quotes) it will show the title of the most recent story which is getting sent out with that email, e.g. you could enter something like «New story on Lara’s blog: %laststory%.