Constant Contact is enables you to send mass emails without being tagged as a spammer. Typical mail servers you use through an ISP won't allow more than 50 emails to go out in a batch, so any larger group would have to be divided into groups of 50 or less and sent out over a period of time. If your sending email address gets tagged as a spam distributor, lets say, by AOL, then in future AOL will not distribute your email newsletter to anyone using AOL for email. And AOL tags senders as spammers if AOL servers receive group emails of 50 or more by "uncertified" senders (which is why ISP mail servers don't let you send more than 50 at a time...). Constant Contact and other listserv providers offering mass emailing services go through a process that "certifies" the mass emails they send as being "legitimate."

In addition to that protection, Constant Contact and similar companies provide the opt-in and opt-out mechanisms and enable easy management of email lists as well as the tracking data.