And since I don't know when, when postfix receives e-mails it stores it with uid postfix and gid postfix whereas it should store with uid postfix and gid as in the field "user_id" of a table in a database.
It had worked but now I don't know why it doesn't anymore.
I tried setting :
(with 1000 an existing gid) but it still stored mails with postfix : postfix.
Have you got an idea ? What do you think I can do to solve this problem ?