I’ve read about Rainloop (another web-based e-mail client, link) before, but it was shot down because it wasn’t available for commercial parties that are using MAIB. It is however a very good webmail e-mail client, not as visually bulky as Roundcube.
I saw that the basic version is available under the AGPLv3 license, which allows for commercial or
non-personal use. So maybe it could be an alternative for Roundcube?