1)i have Registered user who registers with his email id

2)i have paid users

now the registered user gets all the functionality of paid users with some limitations for 180 days,.

now i have to develop an application which shoots a mail(which should be automated) to the reigistered user 30 days before his expiration of 180 days, stating become paid user and gets the full functionality,then another mail shoots up after 15 days then after 1 week then finally 1 day.In the meanwhile if the user becomes paid user then then this autoemail shooter should stop sending mails

HOW TO do that.any idea,any suggestions,Greatly appreciated.