In his now-famous, A Plan for Spam, Paul Graham wrote:

… the spam of the future will probably look something like this:

Hey there. Thought you should check out the following:
http://www.27meg.com/foo

because that is about as much sales pitch as content-based filtering will leave the spammer room to make. (Indeed, it will be hard even to get this past filters, because if everything else in the email is neutral, the spam probability will hinge on the url, and it will take some effort to make that look neutral.)