What does everyone use to send out newsletters, email campaigns?
apsurd: My honest advice: Think like a business. 
DO NOT build your own email system. Why do that when you can use that time to evolve your app?If you can't pay a monthly mailchimp fee with your current model, then the model needs changing.edit: Take heed of Patrick's "marketing evolution" strategies below and realize there are oh-so-many more lucrative things to be working on than coding a custom mailer.