Google AdSense is only for websites but LaunchBit is an online ad network that will help you monetize your email newsletters. You can have regular email campaings, RSS to Email campaigns or even transactional emails.
It’s an online marketplace where newletter publishers, like yourself, can get listed and find sponsors. Your newsletter must comply with CAN-SPAM laws and must be easy for your readers to unsubscribe.
If you are selling your own ads directly, then you can choose any format you like. The cost-per-click ads available to you are 300x250px and 468x60px banner ads. There are also cost-per-click text ads available to MailChimp publishers.