I can only comment on UK law, but I'd also like to give the usual IANAL disclaimer.
There are two types of contract. One is an implicit contract, the other is an explicit contract.
An implicit contract is one provided by existing legal legislation, whereby an action binds you to the terms of that contract, without any specific signing of a contract document. An example of this is a business selling you an item online, in which they automatically become bound to a provider-consumer contract as part of The Consumer Protection (Distance Selling) Regulations 2000. Breaches of this contract can be enforced by law.
An explicit contract is agreed upon by the involved parties, and usually signed. Most explicit contracts have limitations in law, e.g. they cannot be signed by a minor. An example of this is a non-disclosure agreement (NDA), which is bound by civil law.
The disclaimer at the bottom of an email is an implicit contract, but it is an informal contract. Since the contract is sent without the receiver having any ex post facto way to reject it, a legally binding contract cannot be applied here. An informal contract essentially says "we would like you to do this", but you are free to violate it.
However, there are some important things to think about here:
- If you explicitly agree, through statement in a response, to the terms in the contract, you are forming an explicit contract with the same right to legal enforcement as an NDA.
- If you explicitly disagree, the implicit contract is void. You may still be liable to civil proceedings if you distribute corporate secrets covered by a separate law contained in the email.
- If you do neither, but still share the email, you may be liable to civil proceedings due to breach of informal contract. However, the company would have to prove bad faith on your part, which is difficult.
In general, they're not enforcable as part of law unless you explicitly agree to them, and they're mostly for informative purposes. If you're in a situation where this might be a problem, contact a lawyer.