They're free! There is no additional cost for reverse DNS records or queries.
Yes. Once you set the reverse DNS property for your Cloud Service, Azure manages all the DNS delegations and DNS zones required to ensure that reverse DNS record resolves for all internet users.
No. Reverse DNS is an opt-in feature. No default reverse DNS records are created if you choose not to configure them.
FQDNs are specified in forward order, and must be terminated by a dot (e.g., "app1.contoso.com.").
Where the validation for reverse DNS checks fail, the service management operation will fail. Please correct the reverse DNS value as required, and retry.
Reverse DNS is not supported for Azure Websites. Reverse DNS is supported for Azure PaaS roles and IaaS virtual machines.
No. Azure supports a single reverse DNS record for each Azure Cloud Service. Each Azure Cloud Service however can have their own reverse DNS record.
No. Azure Compute services do not support sending emails to external domains.