To get an idea what configuration files may be you using the old-current hostname it helps to grep /etc/ for the old-current hostname, eg:
# grep -ril `hostname -f` /etc/ /etc/postfix/main.cf /etc/hosts /etc/hostname /etc/mailname
Update /etc/hostname
# echo "ahost.ipduh.com" > /etc/hostname
Update /etc/hosts ,eg:
#/etc/hosts #IP Address FQDN Alias1 Alias2 AliasN 192.0.2.11 ahost.ipduh.com ahost
Update /etc/mailname
# echo "ahost.ipduh.com" > /etc/mailname
Update SSH Host Keys
Replace root@old_hostname with root@new_hostname --the last words-- in /etc/ssh/ssh_host_rsa_key.pub and /etc/ssh/ssh_host_dsa_key.pub
Restart the SSH deaemon.
For Postfix update myhostname= in /etc/main.cf and restart the daemon
For ssmtp update hostname= in /etc/ssmtp/ssmtp.conf
Many daemons have some hostname definition in their configuration files that may need to be changed.
If possible restart the system.
Debian Wiki Change Hostname
URI: http://alog.ipduh.com/2013/01/change-hostname-debian.html