[Minor] Reduce score MICROSOFT_SPAM

Many false positive

rules/regexp/upstream_spam_filters.lua

@@ -28,7 +28,7 @@ reconf['PRECEDENCE_BULK'] = {
 reconf['MICROSOFT_SPAM'] = {
     -- https://technet.microsoft.com/en-us/library/dn205071(v=exchg.150).aspx
     re = 'X-Forefront-Antispam-Report=/SFV:SPM/H',
-    score = 10,
+    score = 4,
     description = "Microsoft says the message is spam",
     group = 'upstream_spam_filters'
 }