…They are assumed to be a wise precaution. But they are mostly, legally speaking, pointless. Lawyers and experts on internet policy say no court case has ever turned on the presence or absence of such an automatic e-mail footer in America, the most litigious of rich countries.
Many disclaimers are, in effect, seeking to impose a contractual obligation unilaterally, and thus are probably unenforceable.
Source: The Economist