Message scheduling Courier supports VERPs, multiple recipients per message, and RFC1894-compliant delivery status notifications. Load limiting You can set a maximum number of messages to deliver simultaneously to the same host. This, in fact, is strongly encouraged so that a single nonfunctioning domain does not take up all available delivery slots. Courier's scheduling engine restarts itself automatically, on a regular basis. This helps with memory fragmentation.
Courier tries to restart itself during periods of system inactivity.
Smart organization of the message queue and temporary directories Courier automatically creates subdirectories when necessary, and deletes them when they're empty. When there's a sudden peak in the number of messages added to the queue, directories used to store the messages grow in size to accomodate the additional entries. On many file systems, once those messages are deleted, the empty slack space in the directory is not reclaimed, and actually slows down subsequent directory operations. Courier automatically removes empty directories, reclaiming the slack space. Optional configuration switches allow pretty much every major subdirectory to be relocated anywhere else.