In the diagnostic report, there are 2 issues:
1. domain literals fail the "Hosted Domain Resolution Test":
xxx.xxx.xxx.xxx (Lookup Public DNS...) Error: Could not resolve xxx.xxx.xxx.xxx in DNS. Error: Could not resolve DNS (9003). Fail
This is incorrect and a reverse DNS lookup has to be done (instead of a forward lookup) or to be passed by default with the status "Literal domain"
2. "Postoffice Status Tests" reports a warning on "Default Postoffice" when the setting is "Disabled". This option is desired in this state when multiple postoffices are hosted and you do want the users to login with the full email address (for security reasons), therefore a warining is incorrect.
Would be desired a "global diagnostics result" of Pass/Warn/Error at the top of the report, so you do not have to scroll and inspect all items if you want to do just a quick check that everything is fine.