I am also having the same issue. v 1.6 RC3.
I get outgoing mail to the admin (alerts and such) but no email to the person who posted the ticket. I have checked all autoresponder settings and they are set correctly. The funny thing is it was working yesterday but stopped working today and no changes were made.
My site is not on Cpanel.
Here is the chain of events:
1. User creates new ticket on osTicket frontend
2. user receives message on screen saying a ticket was created and an email will be sent.
3. An new ticket alert mail is sent to the admin and is received.
4. User receives nothing.
5. Admin makes a reply to the ticket.
6. No mail goes anywhere.
Autoreponders are enabled
Alerts are enabled
auto cron is on
in Departments, autoresponders are enabled.
Thanks.