May be Cade wanted to keep the uniform design and any data access whether it is from DB or external systems - he end up creating DAO.
Though i also agree that both Mailing Service and Inventory Management Service should have been handled bit differently.
In my case for emailing-I have used Apache James Mail Server
http://james.apache.org/ which can persist the messages and handle hard & soft bounces, takes care of SPAM, virus protection also.