Thomas_W
Agree, cPanel does this well to give green ticks in the users cPanel email settings to confirm the spf and dkim is correct. If such a diagnostic tool could pair this with checking the email routing (local vs remote) against the actual authoritative name servers, that would be powerful
Not sure if a general diagnostic script is the best place for it, but an important feature and report to have non of the less.
DMARC is a beast of its own, especially if wanting a monitoring panel built in that tracks the responses.
"And show where a customer use the most space at the disk. We can see it of course, but for customers is this too difficult."
+1 Absolutely, a disk usage tool.