My guess is that the users is on a system that gateways mail into
 a PC/LAN based mail system, and that the gateway is constructing the
 headers incorrectly.  So you were absolutely right in directing the
 user towards his/her local postmaster.  If/When the PC mail user sees
 "owner-listname@..." in his/her mail messages, it's likely that the
 gateway is using the SMTP headers to create the PC mail headers.  It
 should be using the RFC822 mail headers instead.  I've seen that exact
 same problem before by the way.  In my case is was a badly configured
 SMTP to Lotus/Notes gateway.  Such gateways *can* work just fine, but
 if they are misconfigured the user will wind up with unrepliable mail.
 
 -jj