It is better to compile sendmail to do not allow run milters
with root privilegies.

Im suggest to add -D_FFR_MILTER_ROOT_UNSAFE.

Cut from libmilter/README:
Note: we strongly recommend not to run any milter as root.  Libmilter
does not need root access to communicate with sendmail.  It is a
good security practice to run a program only with root privileges
if really necessary.  A milter should probably check first whether
it runs as root and refuse to start in that case.  There is a
compile time option _FFR_MILTER_ROOT_UNSAFE which keeps libmilter
from unlinking a socket when running as root.  It is recommended
to turn on this option:

APPENDDEF(`conf_libmilter_ENVDEF', `-D_FFR_MILTER_ROOT_UNSAFE ')