[exim] transport_filter and expansion

Hi,

I'm trying to achieve this:

transport_filter = '${if eq{$acl_m2}{spam}{/usr/bin/spamc -U /var/run/spamd.sock -u $acl_m1}{/bin/cat}}'

But I get:

2006-01-16 00:15:11 1EyH2p-0002Fy-E1 == ingvild@???
<ingvild@???> R=local_user T=maildir_home defer (-24): transport
filter process failed (127) while writing to
tmp/1137366911.H620387P9050.buick.jordet.net: unable to execute command

I understand that this is because of the whitespace in the command. I've
tried every way of escaping I know of, but it seems impossible to have
both the "${if" and the command escaped correctly.

Is this impossible, or does it only seem so?

Best regards,
Stian