A registered letter (Einschreibebrief) is a type of letter with a proof of delivery. If you need to send legal documents, you must send them per Einschreiben.

Registered letters come with a tracking number1. You can follow the letter, and know when it was delivered.

Registered letters require a signature from the recipient. If you don't need a signature, you can send the letter per Einwurf-Einschreiben1.

If you want to receive a delivery confirmation by mail, you must send an Einschreibebrief mit Rückschein.

You can send registered letters at any German post office.