Lucas... segue o caminho abaixo...

https://www.google.com/search?q=how+to+setup+a+reverse+dns+for+smtp+mail+server+from+another+domain&client=firefox-b-e&sca_esv=25ac67c7e3425c2e&ei=_-gPZ6KbFKS65OUPqJzfyQI&ved=0ahUKEwjiloioqJOJAxUkHbkGHSjONykQ4dUDCA8&uact=5&oq=how+to+setup+a+reverse+dns+for+smtp+mail+server+from+another+domain&gs_lp=Egxnd3Mtd2l6LXNlcnAiQ2hvdyB0byBzZXR1cCBhIHJldmVyc2UgZG5zIGZvciBzbXRwIG1haWwgc2VydmVyIGZyb20gYW5vdGhlciBkb21haW4yChAAGLADGNYEGEcyChAAGLADGNYEGEcyChAAGLADGNYEGEcyChAAGLADGNYEGEcyChAAGLADGNYEGEcyChAAGLADGNYEGEcyChAAGLADGNYEGEcyChAAGLADGNYEGEdI03xQ0CJYx0BwAXgBkAEAmAEAoAEAqgEAuAEDyAEA-AEBmAIBoAINmAMA4gMFEgExIECIBgGQBgiSBwExoAcA&sclient=gws-wiz-serp

vamos falando...

Fjo