Because mail marketing is just one aspect of your larger sales and marketing strategy, success can't be measured by the bottom line. The success of direct mail marketing is relative and depends upon your goals.
Industries, such as publishing, that rely on direct mail to solicit subscriptions
For small companies, a successful direct mail campaign should eventually generate enough revenue to cover the costs of the mailing. Those costs would include the list if you bought one, postage, paper, printing, and the labor associated with the mailing. In other words, a successful mailing breaks even.