Generate repeat business from your customers.
Sending email to your client base is a powerful way to use the web. All businesses should advise clients of opportunities that are relevant to them.
If they have purchased from you in the past AND indicated they are happy to receive future information from you, you have a very strong chance of dramatically improving sales and/or customer satisfaction with email marketing.
An example. Working on the web, our staff are regular users of Amazon and The Book Depository. In spite of the fact we know we are likely to get 'hooked,' we happily subscribe to newsletters in our areas of interest e.g. new programming books etc. Guess how many books we have purchased in our small group because of these regular emails from Amazon advertising new items? It's embarrassing. I won't say the total amount but it's a lot.
Consider adding a link on your site, inviting visitors to sign up for a regular newsletter.
At present, we only offer email marketing to clients who have their website with us.