Reverse DNS is done on the IP address…
Reverse DNS is done on the IP address, not the domain name. So as long as the reverse IP lookup works, and returns a domain that then resolves to the same IP address it will be ok.
So you are ok to send as any email address once you have a PTR for the IP address you are sending on. Just make sure you have SPF records for the domains you do send from, since they are checked.