[Exim] rewrite problem

What options do I have to activate to actually see why a
rewrite fails? I have the following rewrite rule:

www-data@??? \
"${local_part:$header_from:}@${domain:$header_from:}" Fr

(to rewrite envelpe-from using user-provided from:)

It fails quite often and I need a way to see why but
debugging seems not that easy to me - is there any "best
way"? There is no easy way for me to use -brw because I
do not know what this XXX program actually does, nobody
knows it.

Thanks,
Alexander

--
Alexander Koch - <>< - WWJD - aka Efraim - PGP 0xE7694969 - ARGH-RIPE