Hiver - Manage Customer Support, Sales and Projects right from your Gmail inbox. Collaborate with Shared Labels, Shared Contacts, email notes, email reminders and email templates

Mailing Group - Listserv - Allows you to run a free Listserv or email group right from your WordPress dashboard.

Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.

Mailman - Mailman is free software for managing electronic mail discussion and e-newsletter lists.

Nylas Mail - The Nylas Cloud API powers your application with email, calendar & contacts features. Built-in features for better email, calendar, and contact management.

WP Mailster - WP Mailster enables you to run a listserv-like email discussion list on your WordPress site.