The issue you have with using a mailing list for marketing is finding a targeted list that is relevant to your market. If you just buy a general list then you may well get a large customer base to market to, but the difference could be like shooting ducks behind heavy cloud cover, and shooting ducks in a barrel!
The more targeted your mailing list is, the higher your chances of success, so don't base your decision on the numbers on the list, try and base it around the type of list it is.