btw, where are you hosting this? Did you mention this was on Virtualbox? If so, is this at your home/office?
Does your home/office have a static ipaddress assigned for the address?
Also, i am not familiar with configuring ptr records in home/office environments…i dont know if perhaps you might actually need to talk with your telecommunications provider about this?
EDIT…
BTW, dont expect too much help from your home/office internet service provider with this…they intentionally dont support home/office based webservers (thats one reason why normally your download speed is 10 times faster than upload speed…they are trying to discourage home webservers).
You should still be able to setup reverse ptr though (lots of offices run inhouse mail servers)