We created this plugin to create the command file needed to ensure that the Postie process to convert emails to posts will run often enough. This is especially useful for low-volume sites.
Although written for our multisite installations, you can use this on single sites too. If you add/subtract multisites, run this plugin again to update your site list.
Make sure that you test the CRON command once before letting it run every xx minutes. Contact your hosting place, or site admins, for help with enabling and testing CRON commands.