How do I change this as reported by mail-tester.com
Your IP address 123.45.678.90 is associated with the domain sub.myhostname.com.
Nevertheless your message appears to be sent from server.example.com.
You may want to change your pointer (PTR type) DNS record and the host name of your server to the same value.
Here are the tested values for this check:
IP: 123.45.678.90
HELO: server.example.com
rDNS: sub.myhostname.com.