Emails are replacing regular mails from the post office. Not only because it is cheaper, since you do not need to buy a stamp, it's also definitely faster. Emails can be sent in as fast as five seconds, depending on the server, anywhere in the world. No doubt emails are being used to distribute newsletters, promotional mails, and other stuff. How then would you be able to round up email addresses to send those marketingmails you have? Here is where an opt-in list comes in.An opt-in list is a directory of email addresses you can send those mails to. Though it seems to be easy to find email addresses, if you are talking thousands in numbers, it is going to be hard. Besides, you have to determine the recipient of the email. You wouldn't want your readers mostly male if you are selling lipsticks, right? A good opt-in list should include the profile of the email address owner.