What needs to be done to resolve this problem?

After a new installation of the Solaris 10 OS, the following message is repeatedly showing up in
the /var/adm/messages file:
Feb 02 10:04:52 postman sendmail[898]: [ID 702911 mail.alert] unable to qualify my own domain
name (postman) — using short name
What needs to be done to resolve this problem?

A.
add a DNS server to /etc/nsswitch.conf

B.
add a fully qualified hostname to /etc/inet/hosts