In Email Marketing, it is important that you do not take for granted clients, and add them into your mailing list without permission.
It is really important that you first ask whether they wish to be on your list or not. It is even better if you can confirm their permission with what is called a double opt-in. When someone subscribes online, it is because they had an interest in your website or your products or even your blog. Sometimes, when they sign up in the store, is because they feel obligated to sign up, because you were in front of them, so make sure you double opt in, when this is the case.
You have to keep in mind that someone can visit your store because they came with a friend, or because they simply came to return something. Usually, when they visit your website is because they are interested in your company, and want more information. A business card that a client gives you does not necessarily mean that the client gave you permission. Make sure to ask before sending out the first newsletter, this will save you lots of headaches.