setting the reverse dns for a email server

Solution 1:

Responsibility for reverse DNS falls to the entity that assigned the address. If you got an address or addresses from your ISP or hosting provider, they should be responsible for giving it a reverse value.

If you got a block of addresses and provisioned the nameserver for the address block to you, then you are responsible for reverse DNS. However it is unlikely that you have been provisioned as the DNS provider for the block without knowing what you were in for.

Solution 2:

The entity (netblock owner) that has authority for the reverse lookup zone (and therefore the PTR record in that zone) is the entity that owns the ip address, which is probably the ISP or entity that you're getting the routable ip address for the mail server from.

In addition, think of a reverse lookup zone as you would any other zone. Who "owns" (has authority for) the zone? That's who you need to go to to have records set up in that zone.