MacOS X will run out of process slots when you increase Postfix process limits. The following works with OSX 10.4 and OSX 10.5. MacOS X kernel parameters can be specified in /etc/sysctl.conf.

/etc/sysctl.conf:

kern.maxproc=2048kern.maxprocperuid=2048

Unfortunately these can’t simply be set on the fly with sysctl -w. You also have to set the following in /etc/launchd.conf so that the root user after boot will have the right process limit (2048). Otherwise you have to always run ulimit -u 2048 as root, then start a user shell, and then start processes for things to take effect.

/etc/launchd.conf:

limit maxproc 2048

Once these are in place, reboot the system. After that, the
limits will stay in place.