The IP setting is used to assign IPs to websites, but not to the mail server. You may be able to modify the domain's DNS settings so that it sets mail.yourdomain.com to point to that IP, but the server will still send from the main IP you specify in the DNS template.
I think this is a limitation more with qmail, than it is with kloxo. I understand what your goal is, but I do not believe it to be possible without modifying the qmail configuration manually.
You can, however, change the IP address used globally for mail, by modifying the DNS template.
My thoughts on this, are put that client in its own DNS template, and have that DNS template use the dedi IP for mail server.