Had the same problem... during the setup, not sure I clicked Enable POP/IMAP email fetch, or it was set, don't remember which... I unclicked it and the PHP mail function works fine
Update - One thing I see that I was doing that may have an impact... so I am not sure if enabling or disabling the above function matters... I was opening tickets with the same email address for the user as for the support desk...
I ran another test creating a ticket with my yahoo email... an email was sent to the user email and another was sent to the support email address...
This would be a good test to try out and see if that is the problem... either way, I am using the PHP mail function...
Hope this helps