The first acknowledgment message you receive from the email@example.com describes how to retrieve old messages. We reproduce part of that message below. Please note that these messages are sent to firstname.lastname@example.org, not to ACR-L@listserv.okstate.edu.
"Contributions sent to this list are automatically archived. You can obtain a list of the available archive files by sending an "INDEX ACR-L" command to email@example.com. These files can then be retrieved by means of a "GET ACR-L filetype" command, or using the database search facilities of listserv. Send an "INFO DATABASE" command for more information on the latter.
More information on listserv commands can be found in the listserv reference card, which you can retrieve by sending an "INFO REFCARD" command to firstname.lastname@example.org."
For more information on lists in general, you can access the following site: http://www.lsoft.com/.