1. This is the full tracking of the parcel:
China Post Register Air Mail Tracking System

Parcel No.RE481496915GR tracking result
2016-05-25 01:46:00.0 leaving 雅典
2016-05-25 01:46:00.0 雅典 export customs cleared
2016-05-27 09:14:29.0 Beijing Transit Station import customs scan
2016-05-27 13:40:30.0 Beijing Transit Station import customs cleared (domestic transit )
2016-06-01 09:11:32.0 Shenzhen Transit Station import customs scan
2016-06-01 13:19:58.0 Shenzhen Transit Station import customs cleared (domestic transit )
2016-06-02 20:55:40.0 arrive 民治dept. delivery office
2016-06-02 21:02:02.0 民治dept. attempt delivery
2016-06-02 23:08:28.0 民治dept. attempt delivery
2016-06-03 08:45:00.0 民治dept. delivered

2. You can track it here also:
http://www.track-chinapost.com/track.php?code=RE481496915GR&fc=0

3. Or even here:
http://track.track-chinapost.com/china-post/en-RE481496915GR.html

4. Or here also:
http://www.trackitonline.ru/?service=track

5. All the above site are saying that the parcel has been delivered. You could contact recipient and give him the full tracking that I provided and tell him the parcel was delivered. Depending on his answer the next thing you have to do is ask Greece Post to do an investigation about the delivery of the parcel. If I'm not mistaken it was registered mail, so basically somebody had to sign for it.

Hope this help