A polished email may feel official, but ending an employment relationship in Germany generally requires written form with an original signature. Planning the physical letter and its receipt is therefore part of planning the date.
Identify the recipient
Check the contract and employer guidance for the correct entity and address. A line manager may be the natural person for a conversation, while HR or an authorised representative may be the appropriate recipient of the formal notice.
Choose evidence you can retain
Possible approaches include personal handover with a dated acknowledgment or delivery by a reliable messenger who can later describe the document and delivery. Registered post has practical limitations and should not be treated as automatic proof of every relevant fact.
Keep a copy of the signed letter and a factual record of when, where, and to whom it was delivered. Avoid putting grievances or negotiation demands into a simple resignation unless there is a considered reason to do so.
Formal validity depends on circumstances. If delivery is contested, deadlines are tight, or the employer is unavailable, ask a German employment lawyer for advice tailored to the situation.