With Department of Posts, a letter to the effect that it has been delivered is sufficient proof of delivery in the court of law. While you might need a dated POD in case of courier to prove the delivery.
Further, if one refuses to take the Registered Letter/Speed post, a simple remark of a Postman 'refused to take delivery' is tenable in the Court, however this cannot be the case with Private Mail Deliverer.
According to the Indian Post Office Act, only department of Posts has the exclusive domain of transmission and delivery of letters, therefore in really nutty case in the present scenario, the Information Providers might use it to it's advantage.
So the thumb rules are as Col. Kurup has mentioned:
- Certificate of posting: No-No
- Private Mail Deliverer: Use POD with receiver dated receipt.
- Speed Post/Registered Post: Both with or without POD is ok.