Receive via POP3 Connector - Event ID 1053

Suddenly one user constantly gets event ID 1053 recorded in the Application Event Log for every message received via the POP 3 Connector.  The message disappears from the ISP inbox but only turns up in Failed Mail folder.  Internal mail exchange is not affected.  The email address appears to be correct.  The source of the email varies from message to message.  Microsoft KB has no reference to event id 1053. Does anyone know what Event ID 1053 means?

 

SBS 2003 w/o sp1, Dell SC800.