In my experience very low. But you should really configure your mailserver to have the advertised hostname match the reverse hostname for the IP ("PTR record")[0]. The chance of getting your mail rejected if those don't match is a lot higher.

[0]: "PTR record" is the wrong terminology for this. The *.in-addr.arpa is a special PTR record to implement a reverse host lookup. The PTR record in DNS has some other functions as well, e.g. service discovery.