the Cloudron mail configuration respect all the necessary security stuff DKIM, DMARC, SPF and does it automatically for you.
While if the Mail Server is fine to receive; to send, you will more relay on the reputation of your IP and neighbourhood; and even by marking perfect with this kind of test which are very legitimate and well seen. You will still might have an issue to send email to people/company who are using the big one like Google Mail, Microsoft, ...
The thing is Google will suggest you to register your domain with their Postmaster services while Microsoft will simply ignore you (I'm might not being update on that part).
To bypass this issue; again Cloudron do well, you could rely on a relay but then again you don't really control who see your email passing in clear on their server.
Because yes the SSL and TLS are only for the transmission; between server.