The IXPLogon::Idle method indicates that the system is idle, allowing the transport provider to perform low-priority operations.
See IXPLogon : IUnknown.
HRESULT Idle(
ULONG ulFlags
);
The MAPI spooler periodically calls the IXPLogon::Idle method during times when the system is idle if requested by passing the XP_LOGON_SP flag in the call to the IXPProvider::TransportLogon method that opened the current session. At times when the system is idle, the transport provider can perform background operations that are not appropriate during other calls, or that need to occur on a regular basis.