I ran into this problem a little while ago.
I found that the problem was the Postfix binds to a port (25) for
sending, Linux links that port to an IP when needed. This means that the
address may change each time a send is initiated (This is my imperfect
understanding of things).
To make sure that postfix used a valid (Google valid) setup I added -o
smtp_bind_address and smtp_bind_address6 to the smtp entry in master.
This seemed to fix the no PTR found problem for me. The alternative is
to add a PTR record for every possible address, not very practical
particularly with IPv6.
I am not sure about the "unusual rate..." but I wonder if it is a
different manifestation of the same problem.
All I can say is that since adding the smtp_bind... I have not had any
problems.
I hope this helps.
