The sendmail program forks to do its job. Each child process has its own process ID number (pid) which it uses when creating queue filenames. Ordinarily, the uniqueness of each pid prevents any two children from creating identical queue names during any one-second interval. But, on fast machines with short pid ranges, there is a risk that one client might exit and another might start within one second, and the second client will be issued the same pid as the first.

On such machines, the FAST_PID_RECYCLE compile-time macro is defined to prevent just such a collision of pid numbers. In general, this compile-time macro is correctly defined for all currently supported architectures. You will need to define it yourself only if your are porting sendmail to a new system. New ports should be reported to sendmail@sendmail.org so that they can be folded into future releases.

If you are running a precompiled sendmail binary, you can use the -d0.10 debugging command-line switch (-d0.10) to determine if FAST_PID_RECYCLE support is defined (if it appears in the list, it is defined).