e-mail marketing

We sent out an e-mail copy of Ken Kewley's postcard today using a free up to a point e-mail service-mailchimp.com. I have been very impressed with what a great job they do. One wonders however whether or not you annoy more people with an e-mail than you stimulate. What do you think about careful e-mail marketing? Iknow I hate the ones that come everyday.