app_mailers_report_mailer.rb

class ReportMailer < ActionMailer::Base
  default from: "admin@hillcrestanimalhospital.co.uk"

  def income_by_month_report(report_results)
    @report_results = report_results
    mail(to: "danny@hillcrestanimalhospital.co.uk", subject: "Monthly Income Report")
  end
end

app_views_report_mailer_income_by_month.text.plain.erb

<% @report_results.each do |month| %>
  <%= month %>
<% end %>