Exchange Event ID: 9646

Event ID: 9646
Type: Error
Source: MSExchangeIS
Description:
Mapi session “/o=First Organization/ou=Administrative Group/cn=Recipients/cn=user”
exceeded the maximum of 500 objects of type “objtFolder”.

1. Open Registry Editor and navigate to:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\MSExchangeIS
2. Right click ParametersSystem and create New -> Key

3. Enter MaxObjsPerMapiSession and confirm creation of the new sub-key

4. Right click newly created key, create new DWORD value called objtFolder and give it a decimal value bigger than default 500.

5. Create another DWORD value called objtFolderView and give it the same value.

There’s no need to restart Exchange server or even MSExchangeIS service. Give it some time and things will settle down. You might need to restart Outlook client to start synchronising folders again.