[cactus_michellej]

Thanks for your personal insight! Switching up your strategy after four years of doing things one way can be scary. Testing, testing and testing the waters however is the only way to know what's best for you and your clients. Fortunately, you saw some positive impacts of the second e-mail, but unfortunately, negative impacts as well. However, industry standards do show that sending bi-weekly is not an overload on your customers if it is something they except from the beginning. As you say, possibly some of your newer clients were more open to the twice a month e-mail then some of your older customers. With that said, you never can tell. You can simply catch a customer on a bad day and instead of pressing delete they press spam.
Additionally, thanks for advice on the days of the week. Industry best practices do also suggest Tuesdays and Wednesdays to be the optimal time to send.