how to capture koi8 in Subject ?

[stfaprc]

The following is an example of what is showing in maillog

sendmail[28988]: m9RDKtaT028988: Subject:=?koi8-r?B?0M/bzMkgxM/WxMk=?=

I have "koi8" listed in blocked_subjects and have the following in the sendmail.mc

LOCAL_CONFIG
F{MRej} /home/blocked_subjects

LOCAL_CONFIG
Klog syslog
HSubject: $>+CheckSubject

LOCAL_RULESETS
HSubject: $>CheckSubject
D{MMsg} Subject problem with .
SCheckSubject
R$*     $: $(log Subject: $1 $) $1
R$* $={MRej}$*  $#error$: 550 ${MMsg} (Word:$2)

So, why isn't the koi8 getting trapped? My other entries in blocked_subjects are working fine.