Messaging Server can divide its work among several executing processes, which in some cases can increase efficiency. This capability is especially useful with multiprocessor server machines, in which adjusting the number of server processes can allow more efficient distribution of multiple tasks among the hardware processors.
There is a performance overhead, however, in allocating tasks among multiple processes and in switching from one process to another. The advantage of having multiple processes diminishes with each new one added. A simple rule of thumb for most configurations is to have one process per hardware processor on your server machine, up to a maximum of perhaps 4 processes. Your optimum configuration may be different; this rule of thumb is meant only as a starting point for your own analyses.
Note: On some platforms you might also want to increase the number of processes to get around certain per-process limits (such as the maximum number of file descriptors), specific to that platform, that may affect performance.
The default number of processes is 1 each for the POP, IMAP, or HTTP service.