Comment # 13 on bug 929471 from Dr. Werner Fink

Could it be that here the DNS/FQDN (Fully Qualified Domain Name) is mixed with
the static hostname which is given in /proc/sys/kernel/hostname?

The first is bind normally to the first (ether)net device whereas the second is
set by systemd-hostnamed.service(8) (compare with man:hostnamectl(1) and
hostname(7)).  In common practice the names before the very first dot do agree
but this is not required. Beside this there also exist YP/NIS domain names
which may or may not agree with the DSN domain name.

       hostname - show or set the system's host name
       domainname - show or set the system's NIS/YP domain name
       dnsdomainname - show the system's DNS domain name
       nodename - show or set the system's DECnet node name

For any MTA the Fully Qualified Domain Name of the host is the one which (IMHO)
should be used as e.g. relay hosts/servers do check for this name.