Redmine

I don't think this should be in redmine core, every mail system does things differently...
We had the same idea and wrote a plugin that uses regex to achieve the replacement (plain_text_body from mail_handler.rb):

module MailHandlerPatch
  def self.included(base)
    base.send(:include, InstanceMethods)
    base.class_eval do
      alias_method_chain :plain_text_body, :notes_cleanup
    end
  end

  module InstanceMethods
    def plain_text_body_with_notes_cleanup
      body = plain_text_body_without_notes_cleanup

      body.gsub! /\(Embedded image moved to file: (.*)\)/, '!\1!'
      body.gsub! /^\|------------>\s*?\n\| ([^|]*)\|\s*?\n\|------------>\s*?\n  >-{80,}\|\s*?\n  \|([^|]*)\|\s*?\n  >-{80,}\|/m, '|\1|\2|'

      body
    end
  end
end

This converts the stuff our Lotus Notes uses to