spam.m4: Skip SpamAssassin check for mail from relay clients.

spam.m4: Skip SpamAssassin check for mail from relay clients.

The expectation is that relay clients use us as their primary path for
delivering mail to external users.  If we're going to allow them to do
that, it doesn't seem right to subject it to spam checks -- particularly
since there isn't a receiving user to set a spam score limit.

No, I'm not completely sure about this.