Here's an interesting article about abusing the relationship with your customers who opt-in to receive email marketing. Just because people opt-in doesn't mean they want to receive messages every day - or every week for that matter. How to avoid this? When people opt-in give them a choice about how often they want to be contacted by you. Also ask what kind of messages they want to get. Newsletters? Promotions? Press releases?
You've worked hard to build up an in-house list - don't blow it by abusing the relationship.
