In order to add an email address to your subscriber list for an eNewsletter, you need to gain explicit permission from the person to whom the email address belongs to.

It is not ok to add all of your Outlook contacts or your current clients to your subscriber list.
It is not ok to all the participants of an expo unless you expressedly ask each person whether they want to receive your newsletter.

To get a clear picture of when you can add a contact to an email list, read AWeber’s “Collect Subscribers Offline: Do’s and Don’ts“.