Reverse DNS is not a valid Hostname error from MXToolBox

Solution 1:

I'm not sure about outright invalid but it's at least highly unconventional to have a bare domain name as the (fqdn) hostname for a host.

Normally one has a hostname, eg zeus and a domain name eg example.com forming a fqdn hostname zeus.example.com.

Other than strange choice of name, it looks like the reverse DNS is set up properly (properly forward-confirmed).

$ dig -x 188.166.32.113 +short                                                                                     
izanami.co.
$ dig izanami.co +short                                                                                            
188.166.32.113
$ 

As a sidenote, your dig +short ptr 188.166.32.113.in-addr.arpa looks up the reverse DNS PTR entry for the IP 113.32.166.188 (hence the wildly different result compared to looking up your IP).