The strongest proof is the provider's own legal cancellation button, because the law presumes a declaration sent that way arrived immediately.
The provider must also confirm the content, the termination date and the date of receipt. Where that route is not available, we build an evidence bundle instead: timestamped screenshots before and after sending, the confirmation page, and the letter exactly as it went out.
A fax transmission report is weaker than people think — German courts do not treat an "OK" report as conclusive proof of receipt.
Keep every confirmation.
Store it somewhere you can reach after you leave Germany.
§ 312k (3) and (4) BGB. General information about German law, not advice on an individual case.
The strongest proof is the provider's own legal cancellation button, because the law presumes a declaration sent that way arrived immediately.