Reverse DNS is not a valid Hostname

Hello,

I installed postfix with webmin on Ubuntu14 LTS. For the reverse DNS, the name of the droplet is mydomain.com.

The result of postconf smtp_helo_name is:

root@Ubuntu-14LTS-NY1:~# postconf smtp_helo_name
postconf: warning: /etc/postfix/master.cf: undefined parameter: mua_sender_restrictions
postconf: warning: /etc/postfix/master.cf: undefined parameter: mua_client_restrictions
postconf: warning: /etc/postfix/master.cf: undefined parameter: mua_helo_restrictions
smtp_helo_name = $myhostname

Why smtp_helo_name reply $hotname, that looks strange.

And I have two warnings with http://mxtoolbox.com

• Reverse DNS is not a valid Hostname
• Reverse DNS does not match SMTP Banner -

In main.cf, I have:

smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
biff = no
myorigin = /etc/mailname
mydestination = mydomain.com, localhost

In /etc/mailname there is mydomain.com

What can I do to fix these warnings?

Thanks, Vincent.