Don't serve tofu when you promised turkey. Make sure you send your subscribers the kind of email they signed up for. If they are expecting a monthly best practices newsletter, don't send weekly product information emails. Respect what they did (and did not) opt into.
You will keep your customers happy, and keep them as loyal email subscribers if you are delivering relevant information to them. Don't get lazy - keep them interested and keep giving them what they want. Let them eat cake!