Do you own a domain name? In that case it would be something like "machinename.mydomain.com". If not, you can just just make something up, say, server.example.com
I don't know KWrite, but if you can open main.cf (it should be in /etc/postfix I would think), there is a parameter in the call myhostname that you can set.
If you have webmin installed, you could use that.
If you set the hostname for your machine, Postfix will pick that up too. I don't know where you do that in Mandrake, but it's probably network settings somewhere.
Don't forget, you'll need to do a
# postfix reload
for the changes to take effect