auto responders

[verve violet]

Hello,
We are trying to develop an application that can create autoresponders ranging from the simplest "just reply to the e-mail received" ones to automatic sequential follow-up machines sending out messages to the prospects/customers of a site at presefined time intervals.
There is a website ,when a user comes to the website site, wants to sign up for something, or buy something, they place their email address and name in a form. When they submit this form, the auto responder we are trying to develop should come into action. They could recieve a personalised email message immediately, or after a set number of days pass... then after a different amount of days pass, the next email should be sent to them. This can continue indefinitely as long as we've written out emails for them to receive.
But don't have an idea about how to send those messages as delayed 'follow-up' emails.Please suggest the best possible approach for this process.
thank u
verve

[Bear Bibeault]

Please do not cross-post the same question more than once. It wastes people's time when multiple redundant conversations take place.