Q:

Is an rDNS PTR record really necessary to avoid spam filters?

I realize this is a somewhat abstract question, but does anyone have any experience with setting up an e-mail server without an rDNS record?
I ask because I really want to host my mail server at home, however my ISP doesn't support creating an rDNS for private customers.
So what I want to know is: How heavily does the rDNS check weigh in on spam checks?
So if everything else is in order, will my mails still be marked as spam simply because of a missing rDNS record?

A:

Is an rDNS PTR record really necessary to avoid spam filters?

Yes. 

Your provider may not support setting a custom PTR record but many providers do set a PTR record and associated A record in their domain for the ip-addresses they issue to customers.  E.g you may have been assigned ran0m-lo0k1ng-hostnam3.dsl.customers.isp.example.com 
Simply ensure that your mail server identifies itself with that hostname and you should be good with regards to reverse DNS checks. 
(Note Since mail servers can host mail for many different domains the hostname a mail server identifies itself with does not have to from the same domain as the sender email addresses in the messages transmitted by that mail server.) 

