It would appear from INFO KEYWORDS:
 
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* Daily-Threshold= <number> *
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This keyword limits the number of postings that may be processed by the list in
a 24 hour period.  The default is Daily-Threshold= 50.  When the keyword's value
is reached, the list is automatically placed on hold, and the list owner or
LISTSERV maintainer must issue the FREE <listname> command.  Note that it may or
may not be advisable to increase this parameter for higher-volume lists -
individual list owners should study the issue carefully before increasing the
daily threshold of their high-volume lists.