WP Mailster - WP Mailster enables you to run a listserv-like email discussion list on your WordPress site.
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
Listeron - An email address for your group. Discuss. Debate. Organise. All with email.
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
Sendy - Sendy is a self hosted newsletter app that sends emails 100x cheaper via Amazon SES
MailClark - The Slack bot for external communications