An entire new app for e-mail.
A few interesting possibilities:
1. Only viewable to certain members, and you can provide e-mail addresses @ your domain to admins, staff.
2. Viewable to anyone. You can either sell e-mail accounts as a service (for example if my community is about tech or special interests. Imagine my community is lefthanded.com and any community member who is left-handed can register for an e-mail address of their very own) - either sell accounts as a service, or award them to patrons who donate so much $$ or even Jcoins.
3. Now that Woltlab Cloud is existing, Woltlab can also offer e-mail hosting in several ways- via the app, via account, as a user or as a reseller. (Of course, if someone uses their own hosting and already has access to email hosting, they can use their own MAP/SMTP to configure it).
4. Community user accounts can be tied and hooked to particular e-mail addresses... or not. An e-mail alias can be specified so that it's not strictly necessary for user Mistah_Q to also have the e-mail Mistah_Q@lefthanded.com but I can have CoolSexyDwarf@lefthanded.com instead, but the user account still links into the e-mail account.
5. Attractive e-mail interface that makes sense within the Woltlab framework. Navigation and folders on the sidebar, easy buttons, etc... I don't know, I'm not the designer. But did you ever try to put rainloop php on a custom WCF tab? Man, it's an ugly headache.
6. Seamless interplay with Calendar, of course.