A nice addition to the massmailer plugin would be that it sends out an automatic mail with the newest listings.
Options would be.
1. What days to send out the mail and what time
2. To what subs do we want to send to
3. From what categories
5. Numer of latest listings example, send the last 10 listings
I have login on several classifieds websites that uses this method and gains a lot of users back once they make an account.