Inspired by my Yahoo rule and another spam, I tried a similar rule for MSN: 
header   RM_hm_MSNCaps  Message-ID =~ /<[A-Z]+\@[a-z]+\.[a-z]{2,4}>/  # no /i
describe RM_hm_MSNCaps  Ratware all-caps message-id 
score    RM_hm_MSNCaps  1.330
#hist    RM_hm_MSNCaps  Created by Bob Menschel May 15 2004

OVERALL     SPAM      HAM     S/O   SCORE  NAME
  93101    71307    21794    0.766   0.00    0.00  (all messages)
   9429     9422        7    0.998   1.00   1.33  RM_hm_MSNCaps

OVERALL%   SPAM%     HAM%     S/O    RANK   SCORE  NAME
  93101    71307    21794    0.766   0.00    0.00  (all messages)
100.000  76.5910  23.4090    0.766   0.00    0.00  (all messages as %)
 10.128  13.2133   0.0321    0.998   1.00    1.33  RM_hm_MSNCaps

My 7 ham hits are all in year 2000 and 2002 -- nothing since Jan 2003. Looks 
like another good candidate, hitting 13% of all spam in my corpus. 

I had another spam, I'm guessing from the same ratware, matching 
ALLCAPS@hotmail.com, and tested that as a rule, but hit no spam other than the 
one.