It has most of the features which a user requires and that too for free,the main being (track,schedule mail for future...) .One can also get a reminder if the receiver doesn't reply.While the paid web app works with mobile and has recurring reminders.Works well to send to y,h and g(yahoo,hotmail...) as i tested it. Generally i study similar applications of the same category for updates,features & easy usage.For this i studied toutapp,followup.cc,nudgemail,lettermelater and most of the results one gets in searching google for the same. boomeranggmail(dot)com Regards.