[Herts] How do I qualify my hostname please?

[Lexx]

I've had a similar error before. I'd like to get this fixed now so that
I can send mail via scripts

What do I have to do to qualify the name in /etc/postfix/main.cf 
What difference does it make if it is qualified or not and why was this
not done automatically?

Many thanks in advance

===========
last: /var/log/wtmp: Permission denied
sendmail: warning: My hostname saphire is not a fully qualified name -
set myhostname or mydomain in /etc/postfix/main.cf
postdrop: warning: My hostname saphire is not a fully qualified name -
set myhostname or mydomain in /etc/postfix/main.cf
===========

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