domain name system - Reverse DNS (PTR) for Azure VM

I'm running an Azure VM (Classic) that hosts an email server. Some domains are not allowing my sent emails due to missing reverse dns/prt record.

I tried to follow this guide: https://azure.microsoft.com/en-us/blog/announcing-reverse-dns-for-azure-cloud-services/

I have a custom domain (say mail.mydomain.com) mapped to the IP and I tried to add a reverse DNS with:

Set-AzureService –ServiceName "mycloudservice" –Description "Reverse
DNS for mailserver" –ReverseDns Fqdn "mail.mydomain.com."

But I get the following error:

Set-AzureService : BadRequest: The reverse DNS FQDN

telemetry.yara.com. must resolve to one of: a). the DNS name of thi s
Hosted Service (xxxx.cloudapp.net), b). the DNS name of a different
Hosted Service in this subscription (a4684608-5
4c0-4c96-b42f-daf646401c58), c). a Reserved IP belonging to this
subscription, or d). the IP of a deployment or of a VM in this
subscription.

Note that this VM also has an instance IP (long story short: we need to ping the IP) and the domain is mapped to the instance IP and not the virtual public ip. Can that be the cause of why I can't add the PTR?

Any ideas on how to add the PTR while still having the domain point to the instance ip?

• Instance IP address
  An instance IP address is a public IP address that can be used to access virtual machines in Azure. Unlike a VIP, each virtual machine in a domain name can have its own instance IP address. Additional charges may apply when using public IP addresses.