I believe an important purpose of disclaimers is to put the receiving party on notice. This is really a "save your behind" provision in case something bad happens. However, if the sending party is a professional services provider (legal services, financial services, tax services etc), then the disclaimer also serves the purpose of reducing liability. Emails (especially short ones) usually don't have all the caveats and conditions associated with proper legal advice (for example). To ensure that people don't treat such emails as full fledged legal advice, these disclaimers can be essential.
That said, a disclaimer is very simple to write if you are aware of the kind of notice you want to give to the reader of the email. Here is a sample notice for legal services provider.
Notice: This email (including any attachments) may contain legally privileged and/or confidential information and is intended solely for the recipient(s) named above. If you are not the intended recipient, you are hereby notified that taking any action on this information is strictly prohibited. If you have received this transmission in error, please immediately notify the sender by return email and then destroy all copies of the transmission.
I am not an attorney, but work with them on a daily basis. I have seen more than a handful variations of the above notice. In the above one, I am not sure if there is law around taking action on information by an unintended recipient. Also, the above notice is silent on the consequences if any unintended recipient does not notify the sender.