Opt-in e-mails are targeted and often personalized and carry information about specific topics or promotions that users are interested in learning about. Typical opt-in e-mails contain newsletters, product information or special promotional offers. For example, if a user frequented a Web site that sold books and music online, that user could "opt in" to receive announcements when his favorite author or musician released new material. The promotional e-mail may even present the recipient with a special promotional offer to purchase the product at a discount available only to those on the opt-in list.

Spammers, however, have found a way to use the opt-in feature to their advantage. Spam e-mails often come with opt-in features in fine print or obscured somewhere in the body of the e-mail with text that reads something to the effect of "Failure to respond to this e-mail will automatically opt-in the recipient to future mailings." The strategy of responding to the spam and asking to be removed from future mailings also has critics as some claim that responding to a spam e-mail with a request to be removed from the mailing list signals that the e-mail address is a working address and the address can then be passed on to other spammers.

Also see Getting Rid of Spam in the Did You Know section of Webopedia.

Internet.com's Opt-In Announcements  
Free targeted third-party mailings that will notify you of products and services that meet your technology interests.

ROKSO Home Page 
Database of known spammers who have repeatedly violated ISP's anti-spam regulations.

8 Ways to Take Names
Some creative, but ethical, methods for getting visitors to opt-in to your e-list.

The Spamhouse Project
An organization that works with ISPs to identify and remove spammers from the Internet.

Eye on Spam
Powerful weapons have emerged in the war against spam, so why isn't the problem going away? A 2002 Datamation article.

How Not to Unsubscribe
Is it safe to use an 'unsubscribe' link to stop getting e-mail from a particular sender? Or will clicking the link just get you more spam?

Opt-In E-Mail: The Solution to Today's Mail Scares
InternetDay looks at the financial advantages of opt-in e-mail programs.