A post is coming on this topic soon.. But based on what I have seen the short answer is direct advertising, ecommerce, or paid subscription.

Thanks Chris. (I'm running a brand new niche email newsletter for patent attorneys and would enjoy chatting if you have the time. My email is in my profile.)

Can't see your email, make sure to put it in your about section (can't see it otherwise). Send me a note, mine is in my about section.

From the few examples I've had the numbers on in this field, their CPMs are sky high compared to Web advertising, so having people "sponsor" certain mailings can pay off in a way it doesn't on the Web.