Retrieve Campaign Lift Letter Content

Renders one of the campaign's lift letters as auto responder content: the lift letter's body with its offer links pointed at the campaign's tracked effort on the sending publication's domains. Pass the returned content along with lift_id when creating an auto responder (POST /messages/auto-responders) or a variant (POST /messages/auto-responder-variations), with the variant's template chosen as usual. A variant with a lift_id is rebuilt automatically whenever the lift letter's content changes. subject and preview_line are the lift letter's, to start the variant's from.