Comment # 31 on bug 929471 from Dr. Werner Fink

(In reply to Dennis Golden from comment #30)

The hostname/domainname used in uname(1), set with sysctl(8) or
gethostname(2)/getdomainname(2), and accessible at /proc/sys/kernel/hostname or
/proc/sys/kernel/domainname has nothing todo with the full qualified hostname
for the network device of the system which has to be used by a MTA like postfix
or sendmail.

The domainname with is NIS/YP and the hostname is for the local system only!
It is very common to use the same short hostname for the local system as for
the first part, aka without domainname, of the full qualified hostname of the
first network device but the configuration of a MTA should use the name space
of the network device and never the that of the local system!