Mmm.. suspect e-mail disclaimers are the world's current greatest example of why you should distrust consensus.
Everyone doing it because everyone does it.
If disclaimers are any use then given a decade of fairly extensive e-mail use we would surely have some case-law to back that up. All there really is of course that oft-cited case in the wrong dismissing the influence of a disclaimer over other substantive message content. The only thing the specialist megaquid lawyers seem to agree on is that it might, and that's an untested might, be useful to put a brief confidentiality notice at the start of messages that are genuinely confidential i.e. warn me before I read it.
As above (@pctru)if you really have to have one then your very best bet is to appeal to my good nature (i do have one honest!): "please", "would you kindly", warn me about being subject to FOIA etc. If I ever get anything sensitive from a public org with "must", "prohibited" and their ilk then it's going direct to a red-top and not least the ICO because I just hate that lazy facist tat... as in who the Heck do you think you are to give me orders? And I'm reasonably sure the ICO latter will tend to focus on the internal policies and procedures you don't have to help prevent it happening in the first place whilst contemplating a fine... "doh, I thought putting some threatening psuedo-legalese on every e-mail anyone sends would.." isn't going to cut it.