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You should always 'sell' this option. Offer a reason why visitors would want to be on your list. This could be an offer of periodic discounts or news regarding the latest products you have in stock. You might even offer an instant discount to encourage new sign ups. You should also have a sign up field that allows members to decide what format they prefer the emails. This would be either plain text or html.
What can you do with your mailing list once you have compiled it? Anything you want - within reason. However, if you abuse your list you are likely to annoy your customers and risk having them opt-out of the list.
Common uses for a list would be to advise customers of an offer or discount you are running. Sales and clearance items may also be announced in this manner, however it is advisable to do so in one email to cover a broad range of issues. Telling customers that you have new items on sale or in your range may also be a suitable use.
The concept of 'less is more' applies to business emails. They should be short and to the point. If graphical or HTML mails are sent, then including such images as your store header, and an image of an item or two that are on sale.
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