The QMMEX logs may report MAPI_E_LOGON errors at random when many agents are running simultaneously and connecting to Server 2008 GCs. These errors occur when trying to log onto a mailbox, however permissions are correct and the logons often work correctly.
This problem may occur if the default limit of 50 NSPI connections by a single user to a given Server 2008 GC is reached. This limit was introduced in server 2008 to limit the resources a single user could use on a GC. Many agent instances running concurrently may exceed this limit.
To resolve this issue, increase the limit in the registry of the GC. Instructions can be found in the following Microsoft KB article: