Off line business owners and Internet business owners are using email marketing opt in list building. Both are finding it profitable to market products to customers who agree to receive emails.
This is what makes a list an opt in one. People agree to receive email which is different than sending out spam emails. With email marketing of this type businesses have earned the right to contact their subscribers at any time.
The key is offering helpful information a person would want to continue to receive. If at anytime a person doesn't want to receive emails any longer they can unsubscribe from your mailing list.
This is as easy as clicking a link at the bottom of each message being sent. People have short memories online today.