What salutation do you use if mailing a "mass mailing" to clients who have used your services in the past and do not want to individualize each letter
I'd be guided by the organization's name. Are you looking for donations, or providing information to past and (you hope) future customers of your business? If there's a possibility of using "Friend" or "Member" (the organization is Friends of Jacksonville Library, or Jacksonville Opera) then you can say "Dear Friend of the Library" or "Dear Jacksonville Opera Members." If it's a hair salon or boutique, you might say "To Our Valued Customers/Dear Valued Customer" or simply "To our clients."
Does that help?
[native speaker and writer, not a teacher]