I must review all my contact. Check manually if they still exists and if they are still interesting, delete and if I want to keep it add it.
It is a huge work. The only problem I have is that contact is ordered only alphabeticaly. There is no order by modification or added date or check date or activity date.
I can imagine activity date and I could delete all contact without activty after 3 month or something.
It was just my morning idea.