Nextcloud in MIAB is intended for contacts and calendar. You can also use it as a file store (WebDAV) as I do. The version is also a little behind and MIAB is running on Ubuntu 18.04
If you are experimenting then fine but MIAB is intended as a solid and simple mail server system.
If you are looking for a good mail server set up which also supports extended features of Nextcloud, then cloudron.io is worth looking at.
Mail is similarly easy to set up. Later version of Nextcloud available. Security means apps are run in separate containers. Free version has mail plus two apps.
Digital Ocean has prebuilt images. Cloudron does require more VPS resources than MIAB
I run both. MIAB for my main production mail server. Cloudron for another email domain, Nextcloud and Collabra. Anyway, hope this is of help.