The procmail program is used by Red Hat Linux for all local mail
delivery. In addition to just delivering mail, procmail can be used
for automatic filtering, presorting, and other mail handling jobs.
Procmail is also the basis for the SmartList mailing list processor.
A local mail delivery agent, such as procmail, should be used for maximum compatibility with sendmail.
Courier expects system mailboxes to be in the users' home directories. If your system mailboxes are all stored separately, in /var/spool/mail or somewhere else, you'll need to use a local delivery agent such as procmail.
Courier uses a filesystem lock on mailbox files, Courier does not support old-fashioned dot-locking. If you need dot-locking, use procmail or maildrop (included).