send email for many users

gistfile1.rb

nletter = "...whatever..."
Newsletter.all.each do |user|
    Mailer.deliver_letter(nletter, user)
end

# And your mailer function would look like:
def letter(nletter, user)
    @nletter = nletter
    mail(:to => user.email, :subject => @nletter.subject)
end

def letter(nletter)
    @nletter = nletter
    @users=Newsletter.all
    mail(:to => @users.map(&:email), :subject => @nletter.subject)
end