Let's say something like this appears in /var/log/maillog.

Feb 21 07:53:50 smtp postfix/smtpd[13676]: warning: hostname smtp.example.com does not resolve to address 107.20.59.106: Name or service not known

Of course, this will happen if myhostname in /etc/postfix/main.cf is set to the hostname in the warning.

myhostname = smtp.example.com

I once had this issue where a forward DNS lookup returned the expected hostname and IP address.

~]$ nslookup mail.example.com
Server:         172.31.0.2
Address:        172.31.0.2#53

Non-authoritative answer:
Name:   mail.example.com
Address: 107.22.51.98

But a reverse DNS lookup returned the hostname and IP address in the maillog warning.

~]$ nslookup 107.20.59.106
106.59.20.107.in-addr.arpa       name = smtp.example.com.

In my example, my Postfix server was running on Amazon Web Services (AWS) so I just had to update the Elastic IP (static IP address) to point to the correct reverse DNS hostname.