There are a lot of good an bad ESP's out there. (Email service providers)
And they range from the dodgy end to the highly reputable. The answer to you question depend on where you are operating but my advise is to adopt good practices to build up a more target and useful list to send LESS email and get better results.

Examples are double opt in procedures to verify your list.

A fairly good place for further reading is www.clickz.com.

Like any marketing, we would need to know more about your goals and what your trying to achieve before advising more.