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.
If links and looks are set, begin interacting with the page. Abandon it, adjust the window size, convert. Are error messages appearing when they’re supposed to (if, for example, you don’t input all the form’s required info)? Is your CTA button working? If you abandon the page, are you retargeted with ads? When you resize the window, does your landing page respond accordingly?