Diferent rule for outgoing mail

Hello,

Difficult problem. I want to apply a different spamassassin rule to the
outgoing mail stream. I know about the policy bank MYNETS.
I want to block all the mail to the internet with a specific string in
the subject.
But I do not want to apply this rule to the incoming stream.

I've read about changing the master.cf... but is there an easy way to do
this ?

Greetings... R.Smits