Plunk is an open-source email platform built on top of AWS SES. It allows you to easily send emails from your applications. It can be considered as a self-hosted alternative to services like SendGrid, Resend or Mailgun.
Mailman is free software for managing electronic mail discussion and e-newsletter lists. Mailman is integrated with the web, making it easy for users to manage their accounts and for list owners to administer their lists. Mailman supports built-in archiving, automatic bounce processing, content filtering, digest delivery, spam filters, and more. See the features page for details.