[exim] How to write subjects in to log

Hi All

How can I write the sebject of the mails in to mainlog file?
Must I make some modification in some source file? if where?
I want to try to have a log lines like this:

2005-11-07 11:20:06 1EZ46g-0000mu-AM <= user@??? H=duffy
[10.20.30.40] SBJ="Subject of the mail" P=esmtp S=1639
id=61FB22F0301C4E4EB3412483D15E0807DD5708@???
2005-11-07 11:20:06 1EZ46g-0000mu-AM => user@??? R=dnslookup
T=remote_smtp H=remotemail.someshere.com [111.111.111.111]
2005-11-07 11:20:06 1EZ46g-0000mu-AM Completed

Andrea