🐛 BUG: Getting 404 when fetch api route behind cloudflare dns/proxy

[dekadenns]

Which Cloudflare product(s) does this pertain to?

Workers Runtime

What version(s) of the tool(s) are you using?

3.91.0 [wrangler]

What version of Node are you using?

20.14.0

What operating system and version are you using?

22.04.1-Ubuntu

Describe the Bug

Observed behavior

Cloudflare worker with email handler isn't able to reach an application behind cloudflare dns/proxy deployed in hetzner through fetch.
Get a 404 back, application endpoint is not triggered.

Works fine in the same worker using an fetchHandler and calling the worker URL.
Also works fine, when the application is running locally and the worker is calling the endpoint through a temp cloudflare tunnel.

Expected behavior

Expection is that the fetch request in the emailHandler reaches the application and did not get a 404 back.

Steps to reproduce

Please provide the following:

• A minimal working subset of your worker code

const APP_URL = "https://preview.example.ai/v1/endpoint/"
const TUNNEL_URL = "https://one-two-three-four.trycloudflare.com/v1/endpoint/"

export default {
  async fetch(request, env): Promise<Response> {
    //Works fine, response from the app endpoint
    const response = await fetch(APP_URL);
    return response;
  },

  async email(message, env) {
    //Did not work, response.status = 404
    const response = await fetch(APP_URL);
    console.log(response)

    //Works fine, response from the app endpoint
    const responseTunnel = await fetch(TUNNEL_URL);
    console.log(responseTunnel)
  }
}

Please provide a link to a minimal reproduction

No response

Please provide any relevant error logs

No response