lmtp user lmtp_admins or admins on murder front-end system

[Stephen Ingram]

I'm trying to test an lmtp connection from the MTA using lmtptest. I'm
connecting to a murder frontend system which then lmtpproxy's to the
backend.

On the frontend system I have:

lmtp_admins: lmtp-kerberos-principal

On the backend system I have:

lmtp_admins: lmtp-kerberos-principal

Every time I try to connect using lmtptest -m GSSAPI
frontend.system.com I get: badlogin: x.x.x.x GSSAPI SASL(-13):
authentication failure: only admins may authenticate.

When I switch the frontend configuration to:

admins: lmtp-kerberos-principal

everything works. I'm trying to limit admin access as much as
possible. While the backend is still protected, the proxy is not. Is
this possible to do or is it not a concern any way since it's only a
frontend proxy?

Steve