The date of receipt of the notice of leave by registered letter with acknowledgment of receipt is the one affixed by the Post Office service in the delivery of the letter to its recipient.
Reported by the Judgement, the Constitutional Court confirmed that the leave granted by registered letter with return receipt shall take effect only if it has been delivered to the recipient.
This solution is classical, and I always advise my clients to deliver their leave by a bailiff, especially to my clients when owners want to give clearance to resume (resume housing for themselves or a loved one).
The legal problem also applies to the tenant, however it arises less frequently in practice.