Auto answer email is the e-mail that will be automatically sent to customer when his first message is received and added to Redmine. This is a default (for all projects) template, however can be replaced with Answer template from specific project.
In order to prepare or edit a helpdesk auto-answer email template, go to Administration -> Helpdesk and choose Helpdesk template tab. You can modify its content by using macros.
You can also add some styling to your emails by modifying Email stylesheet (CSS) - you will find this option in the same tab, just below the auto response template. It works with HTML messages only.
In order to enable auto response, go to Project -> Settings, Helpdesk templates tab.