An email opt-in list is different from your good old customer. Most business owners tell me they have a list of emails of customers and emails they have collected over the years. But when we talk about an email opt-in list, this is a tool used in internet marketing. This is a place on your website where people sign up to be on your list. In return you will give them a free report or video or ebook, something that will be useful to readers.
I use AWeber as my list manager. I love it because it's easy to use. AWeber does all the work to collect the names, keep them in order. If someone wants to be removed from the list, AWeber does that for you too. Now you have a list of people interested in hearing from you, hearing about your business. You can now easily send emails to customers letting them know about events, sales or new products.
If you don't have an email opt-in list on your website, you need to go get one!