Q: I really don't have any idea about RBL (SPamHaus) ,can you explain how it is related to osticket??
A: It's not. an RBL is a black list configured at your mail server.
Q: Should i do any configurations to make RBL (SPamHaus) allow emails from osTicket?A: There is nothing that you would do in osTicket. You would contact SpamHaus and get the domain removed from the black listed.