Purpose and design of MTA counters
MTA channel counters are intended for indicating the health and performance trends of your e-mail system. MTA channel counters are neither designed nor intended to provide an accurate accounting of message traffic; for precise accounting, instead see MTA transaction logging. The lack of accuracy in the MTA's channel counters in an inherent aspect of their design; it is not a bug. Specifically, the MTA's channel counters adhere to what Marshall Rose calls the fundamental axiom of management, which is that management must itself not interfere with proper system and network operation by consuming anything but the tiniest amount of resource.
Therefore the MTA's channel counters are implemented using the lightest weight mechanisms available, namely a shared memory section on each system. Channel counters do not try harder: if an attempt to map the section fails, no information is recorded; if one of the locks in the section cannot be obtained almost immediately, no information is recorded; when a system is shut down, the information contained in the in-memory section is lost forever. MTA counters implementation provides further discussion of the implementation of counters.