This is one of those issues that you will need to involve/bribe/blackmail your DNS admin for assistance on as you will want the name your server claims when trying to send to be correct. Mnay servers will bounce your mail if the sender doesn't correctly resolve both forward and reverse.
For example, let's say your server is mail01.example.com. When your server, with the $myorigin correctly configured to advertise as mail01.example.com, contacts my server, my server will do a DNS lookup of of the IP address you're connecting from and will expect the lookup to match the name your giving me. If it does not correctly resolve, then I will sever the connection because you're probably just sending me spam.