First of all, please confirm that it is not sent to the examiner in person, but to the address and receiving department (such as the acceptance office) at the bottom of the notice page of examination opinions. Sending it to an individual has no legal effect.
Secondly, if you send a registered letter, you can check it at the post office. There is a note from the post office as evidence to prove your mailing date.
Thirdly, if it is sent by EMS, you can use EMS to check whether it is signed (only EMS is recognized, and other express delivery is not official contact).
In addition, if it is an electronic application, you can see the acceptance notice and other documents from the CPC client.
It is better to call the relevant department of the patent office. The specific number is published on SIPO official website, which is difficult to dial, but the operator has a good attitude after getting through, and the inquiry they provide is more effective and direct.