Comment by tjames7000

We don't have full headers, unfortunately. I'm not very familiar with SMTP, but to test things out, we ended up running a minimal custom mail server with the nodejs 'net' module to have full control over responses. I think it'd be possible to set something similar up so that the same mail server handles Hide My Email and normal iCloud addresses differently.

  const net = require('net')
  
  const server = net.createServer((socket) => {
   socket.setEncoding('utf8')
   socket.write('220 ...')
  
   socket.on('data', (data) => {
    if (isHideMyEmail(data)) {
     // handle one way
    } else {
     // handle another way
    }
   })
  
   ...
  })
  
  server.listen(25, '0.0.0.0', () => {
   console.log(`Ready for incoming emails`)
  });

That being said, I have no idea how Hide My Email works technically. Maybe the headers would make it clearer.