According to http://docs.cpanel.net/twiki/bin/view/AllDocumentation/WHMDocs/DomainsIps if you enable "Automatically send outgoing mail from the account's IP address instead of the main IP address":
This option allows you to automatically send outgoing mail from your account's IP address instead of the main IP address. Enabling this option will cause /usr/local/cpanel/scripts/updateuserdomains to automatically populate /etc/mailhelo and /etc/mailips, preventing you from using the other 2 options in the Domains and IPs section.
ALERT! Warning: If you turn on this setting, make sure the reverse DNS entries match those in the /etc/mail_reverse_dns directory.
After enabling "Automatically send outgoing mail from the account's IP address instead of the main IP address" /etc/mailhelo and /etc/mailips are still both blank. Needless to say I am not getting the desired effect, email sent from an account with a dedicated IP address is still being sent from the server's main shared IP address.
I have edited /etc/mail_reverse_dns to look like this:
#.#.#.#: main.server.hostname.com
#.#.#.#: domain-one.com
#.#.#.#: domain-two.com
W.X.Y.Zare placeholders for one IP or if they differ. But you can only have one reverse DNS for an IP. And hopefully this is also configured in the DNS that way! – mailq Aug 23 '11 at 19:32