Hi Jama,
Is your client looking for a
mailing list, for potential new clients? Or creating a way to have new potential clients sign up for her/his newsletter?
If looking for a new mailing list click here to learn how to obtain that information for your client.
If looking for a new way to send out newsletters Aweber is a great tool but they do not give you new clients, it is a program that allows you to have people sign up for your newsletter and or other sign ups.