You will want one that manages your email database and has auto-responder capability. Both systems have their own limitations but are far superior then trying to send Email Database out bulk email through Outlook. Before sending email, you need to use a SPAM checker to ensure that the content of your email is not identified as SPAM. If you are using an online service or Email Database bulk email software, it should have a built in SPAM checker. To communicate with your customers by email you need a list of subscribers. To build this list you must request that your customers become subscribers, preferably using a double-opt-in mechanism.
Put a link up on your web site to gather visitors email addresses. If you use Email Database an online service email service, this is usually very straightforward. The user will click on the link and fill in the web form, which is submitted to the service, which then responds with a Email Database confirmation email. When they confirm this email by clicking on a link, they are added to your database. From there you can send bulk emails with ease. They are now your subscribers. You will want to send them information on a reasonably regular basis. Now some pointers here - do not only send sales messages.
Although a special promotion can provide a "valuable" benefit to Email Database the customer by saving them money, it is imperative that you provide information of value that doesn't require them to make a purchase in order to benefit from it. Give your subscribers Email Database beneficial information and they will be keen to receive your email. Send an eZine on a regular basis. If you have valuable content they will be watching their inbox for it's arrival. Carefully limit the amount of pure sales you do using email marketing. How often you send email will depend on your clients. About once a fortnight is usually often enough to maintain contact without offending subscribers.