[exim] Exim+Spamassassin+Gmail issue

Just thought I'd pass this along in case anyone was having a similar issue
with gmail or for some other domain.

When my servers were bouncing messages to gmail.com addresses and had any
SpamAssassin headers, gmail was truncating the message from that line
downward. So the bounce message would end up at gmail as just a bunch of
Received: headers, which gmail tagged as spam. Removing those headers seems
to fix this. I'm not sure which header was causing it (and yes I have a
vaguely good reason to be scanning outbound bounces; don't ask).

Nothing too complicated obviously, but I'll be happy if it saves someone
else 15 minutes of their life.