Ni!Mail is a webmail connecting to an IMAP server and using python/nevow/twisted to provide a clean and fast interface. It uses nevow livepage, a library that uses XMLHTTPRequest, to make it very dynamic.
LIMeTool (pronounced like the fruit), is intended to be a simple and easy interface for managing accounts on an integrated Sendmail/LDAP/Cyrus IMAP installation. It's written in PHP and is realitively easy to configure and expand.
Unlimited organizations, 3 enterprise SSO connections, role-based access control, and pro MFA included. Dev and prod tenants out of the box.
Auth0's B2B Essentials plan gives you everything you need to ship secure multi-tenant apps. Unlimited orgs, enterprise SSO, RBAC, audit log streaming, and higher auth and API limits included. Add on M2M tokens, enterprise MFA, or additional SSO connections as you scale.