Genuine Mails Getting Detected As Spam

I am using Merak Mail server 8.0.3 (Windows). From past 2 - 3 days many of my users are complaining their genuine mails are going to spam. The value set for antispam is 5 i.e. if antispam value is above it is detected as spam else not spam.

But from past few days which ever genuine mail is detected as spam I have found an very uncommon thing in it. It shows '10.4 FH_HAS_X Has X: header'

The SpamAssassin table shows the following information:

Content analysis details: (16.34 points, 5.00 required)

pts rule name description
---- ---------------------- --------------------------------------------------
0.1 HTML_MESSAGE HTML included in message
0.1 HTML_TAG_EXISTS_TBODY HTML has "tbody" tag
2.2 DEAR_SOMETHING Contains 'Dear (something)'
2.4 BAYES_80 Bayesian spam probability is 80 to 90%
0.0 NO_RDNS2 Sending MTA has no reverse DNS
10.4 FH_HAS_X Has X: header
1.1 SARE_HEAD_MIME_INVALID SARE_HEAD_MIME_INVALID Invalid mime version
0.1 SARE_HEAD_HDR_XMS SARE_HEAD_HDR_XMS Message headers used whic