Client receives following error when he tries to add default address through cpanel.

Fatal! Write Failure: /etc/valiases/domain.com . Ignore any messages of success this can only result in failure!

Client receives this error because domainname.com file is missing.

1)
Go to directory /etc/valiases
root@server1[/]# cd /etc/valiases
2)
Create the file domain.com
root@server1[/etc/valiases]# touch domainname.com

3)
change the ownership as per below.
root@server1[/etc/valiases]# chown username.mail domainname.com

It solves the problem.