Even with relay blocking and blacklisting, spammers find ways to get their cheap messages out. Most major sites filter incoming email to look for spam and viruses. Content filters can be programmed to recognize these phrases, and divert them to a quarantine area, where a human can determine whether to forward it to the recipient. Sometimes these systems go overboard. My company's spam filter used to look for triple exclamation points in the Subject field, and one of my gushy friends could never get email to me.
Most systems allow some recipients to turn off content filtering.
For example, some companies disable content filtering for customer support email accounts, so that customers can rant in a spam-like manner if they feel like it. Because the bottom line for marketers is always economic. Marketers shouldn't send spam because it will get their sites blacklisted and damage their brand. Since I spent most of this article discussing the perils of push marketing technology, I might have scared you into thinking that email marketing is a poor option, and should never be used. However, if executed prudently, email marketing is one of the most effective ways to reach your customers.