Some errors in local delivery are considered transient. For example, if the mail.local tries to open a user's mailbox file, and that open fails because the user's mailbox is already larger than is permitted, delivery will fail. In such a failure, the mail is ordinarily left in the sendmail queue awaiting another try later.

On systems with limited resources, it is sometimes better to take a firm stand and disallow any file that exceeds the user's quota. You do this with mail.local by adding a -b command-line switch:

FEATURE(`local_lmtp')
define(`LOCAL_MAILER_ARGS', `mail.local -l -b')

See the Unix online manual page concerning quota(1) for information about limiting users' use of disk resources and about why this error will occur.