DKIM signing problem

[alex]

Hi

I have a problem with messages signed by my server. All messages send 
from any email client(tb, webmail) , fail verification with :
        dkim=softfail (fail, message has been altered)
except messages send from command line (telnet, sendmail).
Software use is: centos 6 (x86_64), postfix 2.8.0/2.8.5, 
dkim-milter-2.8.3-8.el6.x86_64, no content filtering.
I can't find anything in my config that could modify the body of the 
message after is signed.

Any suggestions?

Regards
Alex