Have you tried to setup the DNS records as suggested by the Cloudron dashboard? Mostly from your screenshot this seems like you have to add the Cloudron in question to the SPF for sending out mails and fixup the PTR pointer.
Lastly it appears your IP is on a blocklist, so you should try to get yourself off that by contacting the blocklist provider.
Alternately you could of course also setup an outbound mail relay as @joseph earlier suggested.