Probably the best way to do this is to set the e-smith system name to mail.yourdomain.com, and ask your DNS provider (probably your ISP) to (1) point mail.yourdomain.com to the static IP address of your e-smith server, and (2) set up an mx record pointing mail for yourdomain.com to mail.yourdomain.com.
Then, in the e-smith manager's "hostnames" panel, point www, ftp, etc. to the ISP's server.