This is indeed also a common case to have the email not hosted on Cloudron. However Cloudron will still want to send out emails for this domain. There are two components doing so:the system as such which would send out with email@example.com the apps, if they need to send out app specific notifications. In such a case the address is dynamically generated depending on the app and which domain it is installed on
Currently none of those can be changed to use some customer email address to send out as.
Regarding the DMARC issue, just to be sure, did you check the mail status tab in the Cloudron dashboard for this domain? The UI will attempt to provide guidance or show issues with email related DNS records there.