ispconfig always assigns a default mailserver to all the mx record generated.
It gives a better image when you get a client to give him a mail server with his domain in it.
example: my ispconfig computer is: www.ispconfigdomain.com
my client domain: myclientdomain.com
my client web : www.myclientdomain.com
my client mail server : mail.ispconfigdomain.com
it would much more nice to create a default MX record as:
my clinet mail server : mail.myclientdomain.com
Today I do it manually. It would be greate if it is automatic.