[Exim] Log Missing

Hi all,

I'm running an exim 3.33 on RedHat 7.0 . It's working fine except some
mails can not be logged. One of my clients received an email from outside
and replied it. From my exim_mainlog file I can see the reply mail but not
the incoming message. We only have one mail server face to internet that is
exim. All emails from or to outside must go through this exim mail server.
I don't understand why I can not see some mail through the main log file.
Anyone have any ideas ...?

Thanks for any help.