Listmonk - Send e-mail campaigns from a powerful dashboard. High performance and features packed into one app.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Sendy - Sendy is a self hosted newsletter app that sends emails 100x cheaper via Amazon SES
Roundcube - Web-based IMAP email client
MailChimp - MailChimp is the best way to design, send, and share email newsletters.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.