[Oleg Krylov]

У вас релей на NIX. И были проблемы с аутентификацией. А это событие говорит о положительном исходе. Не факт, что проблемы были на Exchange. Скорее на NIX.
Для справки:
GSSAPI (Generic Security Service Application Programming Interface) is a function interface that provides security services for applications in a mechanism-independent way. This allows different security mechanisms to be used via one standardized API. GSSAPI is often linked with Kerberos, which is the most common mechanism of GSSAPI.
Kerberos libraries are installed by default on Linux platforms. They are also available for most other Unix platforms, but have to be installed separately.
For Windows, GSSAPI offers integrated authentication for Windows 2000/2003 networks with Kerberos. This method utilizes domain accounts, since local accounts are not transferable across machine boundaries.
The GSSAPI authentication method has no user interface (besides configuration). It does not ask anything from the user. If something fails during GSSAPI exchange, the reason for the failure can be seen in the server event log.