You can add the IP address on https://YOURIP/spprotector/mailsetup.php
What about mobile users?
For mobile users it should not be different than for wired users. They must have a mail client on their mobile and that mail client must be configured with a smtp server for outgoing mail. Mobile users should point to the internal mail host for outgoing mail.
If the problem is that the mail host does not have a public ip and mobile users are not connected through the corporate lan, it can only be fixed if the mx record points to a public ip and the mail host is on that ip.