Listservs can go into "hold" status if too many messages are sent or some other technical error occurs.  Typically, a sender will see a message like one of the examples below when this happens.

The distribution of your message dated Thu, 02 Apr 2015 08:44:51 -0500 with

subject "Test" has been postponed because  the TEST list is held. No action

is required from  you; your message will be  reprocessed automatically once

the list owner releases the list.

Daily message threshold (50) exceeded for the LISTSERVNAME list. The
list  has been  held  and will  stop processing  messages  until a  "FREE
LISTSERVNAME" command is received from you. You can change the daily
message threshold by adding a  "Daily-Threshold= nnn" keyword to the list
header.

The owner of the list can clear the hold by sending an email to listserv@listserv.umd.edu with "free <insert listserv name here>" without the quotes in the body of the message as indicated in the message above.  All held messages will then be released and a confirmation message will come to the list owner after the free command is successful.  You can also change the maximum number of messages that can be sent to the list in a single day by including the "Daily-Threshold=<desired maximum number>" in the list configuration.  The default is 50 emails per day.  Below is an example of the confirmation to the free message from listserv.

> free LISTSERVNAME
The LISTSERVNAME list has been freed, nn files have been released.