CustomMailExporter

class CustomMailExporter
  require 'fileutils'

  attr_accessor :service, :target_users, :target_start_date, :target_end_date,
                :path, :errors, :filename_counter, :emails_found

  # Dates should be in Date or DateTime format.  Users should be an array.
  def initialize(service, target_users, target_start_date, target_end_date)
    @service = service
    @target_users = target_users.map(&:downcase)
    @target_start_date = target_start_date
    @target_end_date = target_end_date
    @path = "/mnt/#{service.id}/"
    @errors = []
    @filename_counter = 0
    @emails_found = 0
  end

  def process
    FileUtils.mkdir_p(path) unless File.directory?(path)

    service.metadatum_class.find_each(service.id) do |datum|
      if target_user?(datum.from) && date_in_range?(datum.date)
        write_contents_to_file(datum)
        @emails_found += 1
      end
    end
    return true if errors.count == 0
  end

  def convert_to_email_address(from)
    from.split("<").last.split(">").first
  end

  def target_user?(from)
    target_users.include?(convert_to_email_address(from).downcase)
  end

  def date_in_range?(date)
    date > target_start_date && date < target_end_date
  end

  def write_contents_to_file(datum)
    begin
      path_and_name = path
      if datum.respond_to?(:content_filename) && datum.content_filename.present?
        path_and_name += datum.content_filename.gsub(/[.<>:"\/\\|\?\*']/, "")
        #use gsub for problem characters in subjects
      else
        path_and_name += filename_counter.to_s
        filename_counter += 1
      end

      path_and_name += ".eml" unless path_and_name.include? ".eml"
      File.open(path_and_name, 'wb') do |f|
        datum.content { |chunk| f << chunk }
      end

    rescue => e
      errors << [datum.key, e]
    end
  end
end

Overall, I think it's good. Nice job!