RDNS is irrelevant - it only checks that the IP address maps back to the domain name use.
So: x.x.x.x maps to smtp.hoster.domain then the MS must name smtp.hoster.domain. As an MX always contains the IN A record, no CNAME, that simply means you put in the hoster's / shared domain name there as FQDN that properly maps.
Basically, I run a SMALL hosting operation. Just for myself and some poeple.
THe SMTP incoming / outcoming email gateway is:
In my own domain (company.domain) I Have an MX record that reads "smtpgatway.x.y". Finished - RDNS resolves properly.
So, basically, you need ONE ip for unlimited domains, the rest is something your smtp server does (i.e. knowing how to handle them, put them to separate mailboxes etc.).