Your rule should look something like this:
Apply rule to messages
from users [Inside] the organization
and sent to users [Outside] the organization
[append][Company legal disclaimer: Blah Blah Blah..] using [Arial], [Normal], [Gray], [with seperator] and fallback to [wrap] if unable to apply.
except when the text [Company legal disclaimer:] appears in the subject or the body of the message.
I just showed you our setup, anything in brackets you can change. Just make sure that the exept text is something that you don't think someone would ever write in an email, that is why I used a colon at the end of my except text.
BobSchleicher