Exim4 and mailman error

[Justin Catterall <justin@masonsmusic.co.uk>]

I think my problem is with exim, not mailman. I've spent a good few
hours googling this and also looking at the exim and mailman mailing
lists, I've tried to follow the instructions but don't seem to be
getting anywhere. So here I am!

Debian Sarge, amd64 unofficial port.
Exim4 4.5-8sarge2
Mailman 2.1.5-8sarge5

The error I get when I send something to the mailing list here is:
R=system_aliases defer (-30): pipe_transport unset in system_aliases
router.

My /etc/exim4/conf.d/router/400_exim4-config_system_aliases
(yes, I'm using multiple config files, not a monolithic one).

Contains: 
system_aliases:
  debug_print = "R: system_aliases for $local_part@$domain"
  driver = redirect
  domains = +local_domains
  allow_fail
  allow_defer
  data = ${lookup{$local_part}lsearch{/etc/aliases}}
  .ifdef SYSTEM_ALIASES_USER
  user = SYSTEM_ALIASES_USER
  .endif
  .ifdef SYSTEM_ALIASES_GROUP
  group = SYSTEM_ALIASES_GROUP
  .endif
  .ifdef SYSTEM_ALIASES_FILE_TRANSPORT
  file_transport = SYSTEM_ALIASES_FILE_TRANSPORT
  .endif
  .ifdef SYSTEM_ALIASES_PIPE_TRANSPORT
  pipe_transport = SYSTEM_ALIASES_PIPE_TRANSPORT
  .endif
  .ifdef SYSTEM_ALIASES_DIRECTORY_TRANSPORT
  directory_transport = SYSTEM_ALIASES_DIRECTORY_TRANSPORT
  .endif

There is a pipe defined in there. Any ideas what could be wrong or where 
I should be looking?

-- 
Justin Catterall                               www.masonsmusic.co.uk
Director                                       T: +44 (0)1424 427562
Masons Music Ltd                               F: +44 (0)1424 434362