1) Only send newsletters to people that want to receive emails from you; so only send emails to addresses that have opted in to receiving them – ie. you have obtained permission/consent from the subscriber
2) Remove Inactive addresses that do not generate any responses over multiple mailings after a certain period of time. For instance, if you regularly email to your subscribers and a particular address does not generate any opens, clicks, calls, or purchases.
3) Too frequent is too much and too late is too late. Don’t hound your subscribers with emails (don’t stalk them every 30min with a mail asking them to buy your products). Often, toning down frequency if you are a regular mailer improves response rates.
I hope you find some of these tips useful!