How to read e-mail headers on GMX

Does anyone know how to read email headers with GMX?

Think you will either have to access it via POP (all headers should be preserved) or perhaps forward it to another email system that does let you read headers (although im not sure if this will preserve all headers)

Tried the second and GMX does not include all headers when forwarded.

Not sure I understand what you are tying to do, but take a look at ePrompter. This is a freeware program which peeks regularly into your email accounts (including Hotmail, Gmail, GMX, etc) and downloads headers only (or optionally the entire body), so you can regularly check incoming mail (headers) and even delete mails you don't want before you need to read/download them with your normal mail client. I originally used this when i was connected on slow (modem) lines to the internet to interrogate headers rather than waste time and money downloading whole mails. Nowadays I simply use it as an effective mail monitor.