Is your subscribers list protected?

If you want to have a successful email marketing strategy you have to make sure your list is really optin, that profiles in your list are people that voluntarily subscribed.
You always have to be sure people are not subscribe through robots filling up marketing databases with random emails.

There's only two ways of making sure no robot uses your forms to subscribe hundreds of potential complainers, bounces or spam traps:

First option is the double optin.
Even though it has been recently discussed on a Bill McCloskey Blog Post, the double optin remains a sure way of making sure people have willingly subscribed to your newsletter.
The double optin system is sending a confirmation email to your subscribers with a link to confirm the subscription. Only users clicking on the link will eventually make it to the broadcast list.

Second option is the Captcha
This second option will not block people from subscribing other persons' email but it will block scripts and robots from posting automatically emails in your list. Thi Captcha system will check on the form posting that the user correctly typed in the text shown in an image.

You don't want to under-estimate the risk of these robots posting emails in your list. Being unprotected might lead you to gather a consequent number of harmful emails.