Tracking pixel uses a transparent 1x1 GIF or PNG

Open-tracking pixels only fire when the receiving client treats the endpoint as a loadable image — a 204 response, a redirect, or an image/gif Content-Type served as text/html causes Gmail, Outlook, and Apple Mail to silently drop the request, so your open-rate metric reports zero even when the email was read. Cache-allowed pixels compound the damage by suppressing re-open events, which breaks engagement scoring, re-engagement workflows, and the funnel attribution data that downstream CAC calculations rely on.

Serve a genuine 1x1 transparent GIF from the pixel route with Content-Type: image/gif, Content-Length, and Cache-Control: no-cache, no-store, must-revalidate; record the open event asynchronously so the image response is not blocked by the database write. Implement this at pages/api/pixel/[...params].ts (or the App Router equivalent).

return new Response(pixel, { headers: { 'Content-Type': 'image/gif', 'Cache-Control': 'no-cache, no-store, must-revalidate' } })