It is fairly common for users to inadvertently add a listserv address (ex. facstaff@listserv.stlawu.edu) to their Blocked Senders list, which eventually results in the user being removed from the list after four days due to delivery failures. This article explains how to have a user re-added to a listserv.
Check the Blocked Senders list in either Outlook or Webmail.
Submit a Service Request to have the user re-added to the proper listserv.