I am having this exact problem as well. 
I am using on a 5.1b4 box the following:
 - ezmlm-idx-std-0.40-2.i386.rpm
 - e-smith-ezmlm-idx-0.1.3-19.noarch.rpm
It should work!  The ezmlm-make man page has the following for the -u flag (Notice the last paragraph):
       -u   User posts only.  ezmlm-make sets up the list so that
            posts  and  archive  access  is  restricted  to  sub-
            scribers.  These are addresses subscribed to the main
            list,  the  digest,  or added manually to the address
            database in dir/allow/ which  accommodates  addresses
            from  e.g.  subscribers working from an address other
            than their subscriber address.
            Posts from  unrecognized  SENDER  addresses  will  be
            rejected.   This  is  relatively  easily defeated for
            posts.  More secure alternatives are  message  moder-
            ated  lists  configured with the ezmlm-make -m switch
            (without the -u switch).
            There is no reason to combine  of  SENDER  checks  on
            posts  with message moderation. Therefore, the combi-
            nation of the -u switch with the -m  switch  is  used
            for  a  configuration  with SENDER restrictions (like
            with -u alone), with the difference that  posts  from
            non-subscribers  will  be sent for moderation instead
            of being rejected. This allows the list admin to  let
            non-subscribers  post  occasionally,  as  well  as to
            catch   subscribers   posting   from   non-subscriber
            addresses.