For the ispCP installation you need a “fully qualified domain name” (FQDN) which is not a main domain. You can use a Subdomain as Example:
Your Domain is: domain.de
Your FQDN is: srv01.server.domain.de (or something else!)
It's important that you take only one and every Time you have been asked after it you tell this one!
Now you have to set you new Hostname at /etc/hostname you can do that with this echo command.
After that we will copy the hostname file for the Mailserver as Mailname:
<cli>
echo your.full.qualified-domain-name.de > /etc/hostname
cp /etc/hostname /etc/mailname
</cli>
Now edit the /etc/hosts file and correct the Information's:
<cli>
# Do not remove the following line, or various programs
# that require network functionality will fail.
127.0.0.1 localhost.localdomain localhost
<ip> <myfullqualified.de> <myfullqualified>
# The following lines are desirable for IPv6 capable hosts
#(added automatically by netbase upgrade)
::1 ip6-localhost ip6-loopback
feo0::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
ff02::3 ip6-allhosts
</cli>
Note: You have to Replace <ip> with your IP, <myfullqualified.de> with your FQDN and <myfullqualified> with your Hostname without Domain-TLD(.de)!
Now restart the Hostname Service:
<cli>
/etc/init.d/hostname.sh stop
/etc/init.d/hostname.sh start
</cli>