Multiple Reverse DNS Entries?

Generally what they care about is that the rDNS result resolves back to the original IP. So a typical setup would look like this:

• www.example.com and www.yourdomain.example both resolve to 192.0.2.1.
• The PTR for 192.0.2.1 is myhost1.yourdomain.example.
• myhost1.yourdomain.example resolves to 192.0.2.1.

I believe most spam filters consider that to be an appropriate rDNS configuration.

If, however, you have separate IP addresses for each website and mail server running on your box so that email from example.com and yourdomain.example appear to come from different IP addresses (and that would be a really bizarre email setup), then the forward and reverse DNS for that domain/IP combination should just point back to each other:

• example.com email comes from 192.0.2.2
• PTR for 192.0.2.2 is example.com.
• example.com resolves to 192.0.2.2

You can do it, but it isn't recommended. There might be cases where you'll break things by setting up multiple records.