daknetworks.com

The best guide I've witnessed on this occasion is here:

http://www.techieshelp.com/exchange-2013-add-a-disclaimer/

I'm posting for my own reference.

ADD EXCEPTION

The only other items I would like to add is that there should be an EXCEPTION:

-click MORE OPTIONS.
-select THE SUBJECT OR BODY INCLUDES ANY OF THESE WORDS.
-paste the disclaimer in the box (don't worry if the whole disclaimer doesn't fit).
-click the + (the plus sign).
-click SAVE.

NOTES:
-the disclaimer will be placed directly at the end of the email.
-the disclaimer will only show at the very bottom of the email; underneath any forwards or replies that may be in the email.
-the disclaimer will only show once and will not repeat if the disclaimer already exists. This means it will not repeat on forwarded emails and reply emails.