A pair of deliverability studies produce some stats on deliverability of legitimate email.

Data from Lyris states that 11.5% of legitimate email is not delivered.

Return Path's 2004 data indicate on average 22% of legitimate opt-in e-mail wasn't delivered to user inboxes. That's a 3.3 percentage point increase over the second half of 2003, when Return Path reported 18.7 percent of messages weren't delivered.

More stats in the article.