Just as a bird in the hand is worth two in the bush, existing customers are worth more than potential customers. The same principle applies to mailing lists. They have already bought your products or used your services. They are the people or companies most likely to respond again to your future offers. Further, the faster you are able to contact an inquiry with an answer, the more likely it is that he or she will react positively to your next offer.
Prospects are potential customers.
They are people who have not yet responded to your offers nor inquired about your organization. However, you have reason to believe that they may have a need for or interest in your product or service. Never take a customer for granted. Developing a mailing list of suspect names should probably be the lowest priority for your business or organization. If you can keep and maintain the same data on each of your customers, you can find which common characteristics are processed by your best customers.