What would prevent "From:" and "Subject:" information from showing up on incoming mail that a POP3 client transferred from a TCPware POP3 server?

This could happen if the TCPWARE_SMTP_HEADER_CONTROL logical is defined to be NONE in order to drop the incoming RFC 822 headers. POP3 needs the RFC headers in order to find the sender, subject, and other header information. If the logical is deassigned and SMTP restarted, future messages will contain the information POP3 needs.