You don't really need rDNS delegated to you (and I'm pretty sure that's not what walrus01 meant), just so long as you can set (or have your ISP set) the PTR RRs to match.
Correctly set PTR RRs can (will) make a huge difference (lots of MTAs will reject if FCrDNS fails).
Functionally from an smtp perspective it doesn't really matter if the rDNS is delegated to you, just that it's set correctly... ordinarily it's something you only have to set the PTR record for once, even if you need to manually ask your upstream to do it for the /32 that is your server, get them to set it to mail.whateverdomainname.net
Correctly set PTR RRs can (will) make a huge difference (lots of MTAs will reject if FCrDNS fails).