Salut à tous, j'ai résolu mon probleme en appliquant ces étapes ici http://forums.techarena.in/small-business-server/91276.htm
Let's try this:
1. On your SBS server, open "Internet Information Services (IIS) Manager"
from Administrative Tools.
2. Check "\exchange", it should use Integrated Windows authentication and
Basic authentication.
3. Check "\exchweb", it should use Anonymous authentication. Also, reset
the IUSR_MACHINE password here since you did it on the default website.
4. Check "\exchweb\bin", it should use Basic authentication.
5. Also, in IIS Service Manager, make sure you have 'Read' checked on the
following virtual directories:
\exchweb
\exchweb\bin
6. Check the version of D:\exchweb\bin\auth\owaauth.dll (SP 1 ver:
6.5.7226.0). If yours is downlevel and you have installed Exchange 2003 SP
1, reapply SP 1, or copy this file from another server.
7. Restart IIS Admin and System Attendant services.