I would have reservations about your responsibilities to protect your customers' privacy when you turn over their information to a third party. How does that meet with your published privacy policies?
Some such services have been known to add your list to the list they sell to others or market to for their own clients. I would thoroughly check my responsibilities before turning the work over to an outside entity, and thoroughly check on their own reputation. The software companies that you mention make sure you never have that worry. (disclaimer - I have no specific knowledge regarding the spam laws of any country that prohibit hiring a 3rd party, just my own consumer concerns reflected here)