The following appears in my SMTPS#x log file.  What does it mean?

30 Aug 2017 10:57:51 Receive error: Unknown error [10053]

It means that the SMTP_FORWARD host associated with the LISTSERV SMTP "worker" in question has dropped the connection and that the worker has immediately restored it.  This is normal if there is limited traffic on the connection; most SMTP servers will drop idle connections after a specified timeout period.  LISTSERV always attempts to keep its SMTP_FORWARD connections open because it is inefficient to open a connection, send mail, drop the connection, and repeat if traffic is heavy.  If you are not noticing any specific delivery problems at the same time, you can safely ignore it.