[exim] Conditional scanning done header

I'm using Exim 4 on FreeBSD, and I've go a "scanning done" header placed
on the mail by my gateway server so messages aren't re-scanned by my
destination mail server. However, I'm having some performance issues
(I'm working on tuning things), and I'm wondering if the scanning done
header can be conditional?

Specifically, I'm wondering if I could put the defer ok condition on the
SpamAssassin scanning for the time being, and if that's the case NOT put
the scanning done header on the mail so the destination server will scan
those messages the gateway couldn't scan.

Thanks,
Mike Oliveri
Systems Administrator
UTI Systems, Inc.
moliveri@???