diff --git a/share/examples/etc/make.conf b/share/examples/etc/make.conf
index f63cb261e16e..ef7e913fbfa7 100644
--- a/share/examples/etc/make.conf
+++ b/share/examples/etc/make.conf
@@ -284,3 +284,11 @@
 #SENDMAIL_LDFLAGS=
 #SENDMAIL_LDADD=
 #SENDMAIL_DPADD=
+#
+# Setting SENDMAIL_SET_USER_ID will install the sendmail binary as a
+# set-user-ID root binary instead of a set-group-ID smmsp binary and will
+# prevent the installation of /etc/mail/submit.cf.
+# This is a deprecated mode of operation.  See etc/mail/README for more
+# information.
+#
+#SENDMAIL_SET_USER_ID=
diff --git a/share/man/man5/make.conf.5 b/share/man/man5/make.conf.5
index dddbe506644d..1710b732c14e 100644
--- a/share/man/man5/make.conf.5
+++ b/share/man/man5/make.conf.5
@@ -547,7 +547,18 @@ command when building
 .Xr sendmail 8 .
 .It Va SENDMAIL_DPADD
 .Pq Vt str
-This variable is undocumented.
+Extra dependencies to add when building
+.Xr sendmail 8 .
+.It Va SENDMAIL_SET_USER_ID
+.Pq Vt bool
+If set, install
+.Xr sendmail 8
+as a set-user-ID root binary instead of a set-group-ID binary
+and do not install
+.Pa /etc/mail/submit.{cf,mc} .
+Use of this flag is not recommended and the alternative advice in
+.Pa /etc/mail/README
+should be followed instead of at all possible.
 .It Va WANT_FORCE_OPTIMIZATION_DOWNGRADE
 .Pq Vt int
 Causes the system compiler to be built such that it forces high optimization