The first question is why would you even wanted to build a list of email contacts? The reason is because you can convert them into potential customers in the long term using good email marketing strategies that include the use of promotion and public relations. So now the question is how exact do you increase the list of email contacts?
Then this comes to first understanding why would people submit their email addresses to you at the first place. One of the way to obtain new email contacts is through e-newsletter from websites. You can attract massive visitors into your landing page using highly targeted keywords that are of high search numbers. Using keywords to lead them to your landing page would lets you filter out visitors who appears to be more interested in what you are offering.
Providing value to visitors is an important factor to deciding whether to join your mass mailing or not. Email don’t come easy these days with people’s awareness of how annoying can spam really be. But not directly promoting whatever you are trying to sell and give them consumable and valuable information for their mind. That way you build a long term relationship with them and visitors see you as a more creditable organization that is worthy for their emails.
Alternatively, you can also buy contacts of database from telemarketing companies. They sell in huge collection that varies from different demographic sets. However the price may be out of your expectation. Or you can also manually collect email database from yellow pages or online resources that could take months to do it. Either method would be expensive and time consuming but using the method of e-newletter or free report or any other valuable information to get their contact would be easier. Another way to do it is through the recommendation of friends.
Subscribers can also recommend the subscription to their friends who share the similar interest as them. A perfect example of a successful viral effect would be Facebook.