Hari No, it won't be that. The email is in fact signed, but doesn't always show (my mail server is configured to not send this information) that is is. What's more likely is this
However, none of these are applicable :)
The reverse DNS is valid (points back to the MX record in the DNS zone which is the correct IP<>HOSTNAME PTR)
The domain is also clean according to Google
DMARC verification is also valid
As is DKIM
Clearly, Google's somewhat bizarre algorythm isn't working as well as it should be.