What is the difference between Email-On-Demand and the auto-reply mailbots I have used?

Auto-reply systems, sometime called mailbots, usually bounce back a static message and do not allow for subsequent inquiries, fax transmissions or catalog generation from a document database. Typically they are used to respond to email messages when a specific person is absent (e.g. "I will be out of the office until March 6 and will not be available to respond to email messages") or in applications where a simple reply is adequate (e.g. "Thank you for your inquiry, we will be contacting you within 24 hours").