HaxeFlixel is an open-source 2D game engine that allows developers to create cross-platform games using a single codebase. The document discusses using email marketing to raise funds for further development of HaxeFlixel through a Patreon or Kickstarter campaign. It proposes sending a series of emails to HaxeFlixel's 950 community members to generate around $6,000 in donations to allow the founder to focus full-time on redeveloping the game engine. The email campaign goals include delivery, open, click-through, and unsubscribe rates that meet industry benchmarks.