The first autoresponders were created within mail transfer agents that found they could not deliver an e-mail to a given address. These create bounce messages such as "your e-mail could not be delivered because..." type responses. Today's autoresponders need to be careful to not generate e-mail backscatter, which can result in the autoresponses being considered E-mail spam.
When a subscriber is sorted into a segment, it can trigger an automation to send to them. Each person’s interactions with your email campaigns or your website can trigger a sequence of follow-up emails based on their interests, allowing you to hone your message to your targeted audience. For example, if someone visits your pricing page, you know they’re probably further down your sales funnel and will want to follow-up appropriately. Or if they went to a specific product page or clicked on a link for that product, you can send additional information about the product, testimonials and more.