The official cloudron demo warns here to not install nextcloud apps. https://docs.cloudron.io/apps/nextcloud/#email
This is kind of fine for Rainloop etc. which are integrated (with differing perfection) into cloudron itself anyway.
But.. the Nextcloud Mail App has the label "Featured" and I think cloudron should think about supporting these featured apps.
If it is too much hassle to suppurt all featured apps, I am most interested in the Nextcloud Mail app. The reason is, that not that many cloudron admins will host mail accounts on their cloudron servers and only in this configuration an almost seemless integration into cloudron is possible. All external mail needs to be manually configured in some ways before first use and my experience is, that most users are fine with this. For them it's more important, that the integration itself (after the first setup) is good, an as most users spend their time in nextcloud nowadays, it appears, it's much more natural for them to have the mailclient inside nextcloud as well (although the UI is, IMO, a crime).