Murder and Backend Authentication

I'm working on getting a Murder setup and I can authenticate and pull mail
directly from the backend server.

However, when I try to proxy the connection I get this in /var/log/messages
on the proxy/master:

Jan 31 13:40:35 zeus pop3[5437]: login: SERVER[192.168.247.241] hbeatty
plaintext
Jan 31 13:40:35 zeus pop3[5437]: couldn't authenticate to backend server: no
mechanism available
Jan 31 13:40:35 zeus pop3[5437]: couldn't authenticate to backend server

I get this in /var/log/imapd.log on the backend server:

Jan 31 13:45:01 draco pop3[32718]: accepted connection
Jan 31 13:45:01 draco master[32724]: about to exec /usr/cyrus/bin/pop3d
Jan 31 13:45:01 draco master[32688]: process 32718 exited, status 0
Jan 31 13:45:01 draco pop3[32724]: executed

With this in mind it would seem that when using the proxy the authentication
method is different somehow. Is this correct?