Incorporate mailing list feature into wordpress site

My client would like to incorporate a marketing mailing list feature on the the company website using an appropriate Wordpress plugin. The plugin would need to be able to handle sending marketing messages to those who opted in. The website uses a custom built theme and has two "Contact 7" forms. One for general inquiries and one for website visitors to make appointments. Ideally, the visitor could check off a box to join the mailing list while filling out the appointment form and contact form. The join email list feature would probably also have to be available on other pages on the left side under the existing navigation links. The job involves: Finding the most appropriate plugin Integrating the plugin to the custom theme If possible, having an option to join the mailing list checkbox on the existing email contact forms. If you have any questions, please let me know. Thank you.