Cypht with Cyrus IMAP and http://sabre.io/ is the base of WikiSuite's mail stack. As of 2019-09, it is far less feature-complete than Kolab, but it is more tightly integrated. And over time, it will become feature-rich, as is the nature of Tiki's community and development model: Tiki is the Free / Libre / Open Source (FLOSS) Web application with the most built-in features. Please see https://dev.tiki.org/Email-as-a-first-class-citizen
Cypht has what you'd expect from a webmail client, and some very interesting features like aggregating email from multiple accounts.