Redmine

??? Learn about Action Mailer?
Your question is amazingly detail-less...

My problem is how to use the Action Mailer of Redmine in my plugin because when I use an other Action Mailer, I have to configure both of them(it is not good).

def new
      begin
      @nouvelle_remarque = Remarque.new
      @nouvelle_remarque = Remarque.create(:project_id => @project.id ,
                                 :version_id => params[:version] ,
                                 :assigned_to_id => params[:assigned_to_id] )
      if @nouvelle_remarque.save
        flash[:notice] ="ok" 
        #send e-mail
      else
        flash[:error] ="ko" 
      end
end
end

I think you have to extend Redmine Mailer and add your own methods.