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 ')
added to sendmail-8.12.7-3 or newer
Florian La Roche