Duncan Findlay notes:
I was experiencing a munging problem with spamassasin and exim -> procmail (without fetchmail). For me the problem was in exim.conf.
To fix this, in the transports section, in the procmail_pipe section,
command = "/usr/bin/procmail -d ${local_part}" |
should be changed to
command = "/usr/bin/procmail" |
also ensure that in the same section,
user = $local_part |
This fixed the problem for me. Hopefully this might help someone else.