State your proposal, and what the other party will get from the venture. You must avoid making it look like a mass email, so use the prospects name, mention their website, etc. A brief overview of your company or organisation, your activities, website URL etc, is necessary. Convince him that this is a good mutual opportunity that he must not pass over. People are naturally suspicious of JV proposals where it looks as if the other party does not benefit, or covers it up.
Joint ventures are a powerful, widely used tool.