deliverquota delivers mail to a maildir taking into account any software-imposed quota on the maildir. This manually-enforced quota mechanism is described in the maildirquota(7) and maildirmake(1) manual pages. Instead of setting up your mail server to deliver the message directly to a maildir, configure the mail server to run the deliverquota program in order to deliver the message, and specify the location of the maildir as the argument to deliverquota.
The second argument to deliverquota is optional. If present, quota specifies a new quota setting for this maildir. Specifying quota is equivalent to running maildirmake(1) with the -q option. The second argument to deliverquota is considered obsolete, and may be removed in the future.
deliverquota reads the message from standard input and delivers it to maildir. maildir may specify a path directly to a maildir folder, not the main maildir (as long as the folder was created by the maildirmake(1) command.
The -c option automatically creates the maildir, and all missing parent subdirectories (as 'mkdir -p'). This option should be used with caution.
The -w N option places a warning message into the maildir if the maildir has a quota setting, and after the message was successfully delivered the maildir was at least N percent full. The warning message is copied verbatim from /etc/courier/quotawarnmsg with the addition of the "Date:" and "Message-Id:" headers. The warning is repeated every 24 hours (at least), until the maildir drops below N percent full.