Hesk Help Deskdesk Tickets

[Hosk]

Does anyone else have Hesk Help Desk Ticket php script installed on their VPS?

I have latest version 2.1 installed , just cannot figure out why 3rd party domains do not receive
admin/staff reply emails.

Ive tested 6 different domain names on different ISP's.
Most (not all) receive the immediate ticket # notification - NONE receive Admin /Staff ticket reply email.

I have complete success with my local VPS domain name email, ie: test@mydomain.com also success with Gmail, HotMail, Live.

Support staff have checked PHP settings & PHP mail() function configuration- reported no problem.
Ticket ID: XCM-533702

HESK uses the standard PHP mail() function

Hesk troubleshoot notes:
1/Prevent the user "nobody" from sending out mail to remote addresses (PHP and CGI scripts generally run as nobody if you are not using PHPSuexec and Suexec respectively.)
THIS IS UNCHECKED on Cpanel

I have not changed any default settings on the VPS, other than a Cpanel password change
and firewall activation prior to Hesk install.

Ideas anyone ?

Thanks.